examples: build_system: demonstrate building app with multiple configs

This example shows how to use ESP-IDF build system features to build
multiple configurations of an app from the same source files.

Configurations are set by overriding SDKCONFIG_DEFAULTS variable when
invoking idf.py.

Related: https://github.com/espressif/esp-idf/issues/5658

examples/build_system/cmake/multi_config/.gitignore

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+build_prod1/
+build_prod2/

examples/build_system/cmake/multi_config/CMakeLists.txt

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+cmake_minimum_required(VERSION 3.5)
+
+# In this example, sdkconfig file is placed into the build directory.
+# This allows building development and production configs side by side,
+# without having them influence each other.
+set(SDKCONFIG "${CMAKE_BINARY_DIR}/sdkconfig")
+
+include($ENV{IDF_PATH}/tools/cmake/project.cmake)
+
+project(multi_config)

examples/build_system/cmake/multi_config/README.md

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+# Multiple Build Configurations Example
+
+(See the README.md file in the upper level 'examples' directory for more information about examples.)
+
+This example demonstrates how to build multiple configurations of a single application. This can be useful in the following cases:
+
+* Building binaries for multiple similar products from single codebase
+* Building the application for development or production hardware
+* Optimizing the application differently for development and production
+
+This example contains three build configurations:
+
+* Development configuration, described by `sdkconfig.defaults` file. This configuration is used by default if the application is built using `idf.py build`.
+* Production configuration for product 1 ("Blinky Smart Light"), described in `sdkconfig.prod1` file. This configuration is not built by default, however it can be built as shown in the next section. It is used together `sdkconfig.prod_common`, common configuration file for all products.
+* Production configuration for product 2 ("Blinky Smart Switch"), described in `sdkconfig.prod2` file. Differs from `prod1` configuration only in product name.
+
+For each configuration, a few configuration options are set:
+
+* Project-specific Kconfig options, `CONFIG_EXAMPLE_PRODUCT_NAME` and `CONFIG_EXAMPLE_FUNC_IMPL`. These options are declared in [component Kconfig.projbuild](main/Kconfig.projbuild). These are used to demonstrate how to create and set project-specific options. These options are set differently in `sdkconfig.defaults` and `sdkconfig.prod_common` files.
+  - `CONFIG_EXAMPLE_PRODUCT_NAME` is a simple `string` option. It is used to set the product name.
+  - `CONFIG_EXAMPLE_FUNC_IMPL` is a `choice` option. It is used to select which of the two source files, [func_dev.c](main/func_dev.c) or [func_prod.c](main/func_prod.c), is compiled and linked. See [component CMakeLists.txt file](main/CMakeLists.txt) for related logic.
+* ESP-IDF configuration options, `CONFIG_COMPILER_OPTIMIZATION_SIZE`, `CONFIG_BOOTLOADER_LOG_LEVEL_NONE`, `CONFIG_LOG_DEFAULT_LEVEL_NONE` are set in `sdkconfig.prod_common` to illustrate a typical production configuration where log messages are disabled and optimization for size is used.
+
+## How to Use Example
+
+### Development build
+
+In this example, Development configuration is specified in `sdkconfig.defaults`, so it is the default one. You can build the project as usual:
+
+```
+idf.py build
+```
+
+To flash the project and see the output, run:
+
+```
+idf.py -p PORT flash monitor
+```
+
+(To exit the serial monitor, type ``Ctrl-]``.)
+
+### Production build
+
+To build one of the Production configurations, specify a different build directory and `SDKCONFIG_DEFAULTS` file. For example, to build `prod1` configuration:
+
+```
+idf.py -B build_prod1 -D SDKCONFIG_DEFAULTS="sdkconfig.prod_common;sdkconfig.prod1" build
+```
+
+* `-B build_prod1` sets the build directory to `build_prod1`
+* `-D SDKCONFIG_DEFAULTS="sdkconfig.prod_common;sdkconfig.prod1"` selects `sdkconfig.prod_common` and `sdkconfig.prod1` files to be used for creating app configuration (sdkconfig), instead of the usual `sdkconfig.defaults`. See the section below on how these two default configuration files are combined.
+
+To flash the project and see the output, run:
+
+```
+idf.py -B build_prod1 -p PORT flash monitor
+```
+
+Note that it is not necessary to repeat `-D SDKCONFIG_DEFAULTS=...` option once the build directory has been created and `sdkconfig` file generated. For example, to build the project again, run:
+
+```
+idf.py -B build_prod1 build
+```
+
+To build and run the app with `prod2` configuration, repeat the steps above, replacing `prod1` with `prod2`.
+
+### Combining multiple files in `SDKCONFIG_DEFAULTS`
+
+`SDKCONFIG_DEFAULTS` build system variable selects the file which contains the default app configuration, used when no `sdkconfig` file is present. If not specified, `SDKCONFIG_DEFAULTS` is set to `"sdkconfig.defaults"`.
+
+`SDKCONFIG_DEFAULTS` can be set to a different name from the command line, using `-D` flag of `idf.py`, as shown above. It can also be set from the project CMakeLists.txt file, before `project.cmake` is included.
+
+It is possible to specify multiple files in this variable, separating them with semicolons. In the example given in the previous section, this is used to create a common config file for production builds and product-specific config files:
+
+* product 1: `sdkconfig.prod_common;sdkconfig.prod1`
+* product 2: `sdkconfig.prod_common;sdkconfig.prod2`
+
+This way the common options do not need to be repeated in each of `sdkconfig.prodN` files.
+
+### Generated `sdkconfig` file
+
+In this example, `sdkconfig` file is placed into the build directory, instead of the project root directory as it is done by default. This allows development and production builds to exist side by side. The location of `sdkconfig` file is set using `SDKCONFIG` variable in [project CMakeLists.txt](CMakeLists.txt) file.
+
+## Example Output
+
+### Development build output
+
+```
+I (310) cpu_start: Starting scheduler on PRO CPU.
+I (0) cpu_start: Starting scheduler on APP CPU.
+This app is built for running on: Blinky Development Board
+func() from func_dev.c (Development) called.
+See README.md for building and running other app configurations.
+```
+
+### Production build output
+
+When building with `-DSDKCONFIG_DEFAULTS="sdkconfig.prod_common;sdkconfig.prod1"` option:
+
+```
+This app is built for running on: Blinky Smart Light
+func() from func_prod.c (Production) called.
+See README.md for building and running other app configurations.
+```
+
+When building with `-DSDKCONFIG_DEFAULTS="sdkconfig.prod_common;sdkconfig.prod2"` option:
+
+```
+This app is built for running on: Blinky Smart Switch
+func() from func_prod.c (Production) called.
+See README.md for building and running other app configurations.
+```
+

examples/build_system/cmake/multi_config/main/CMakeLists.txt

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+idf_component_register(SRCS "multi_config_example_main.c"
+                       INCLUDE_DIRS ".")
+
+if(CONFIG_EXAMPLE_FUNC_IMPL_DEV)
+    target_sources(${COMPONENT_LIB} PRIVATE "func_dev.c")
+elseif(CONFIG_EXAMPLE_FUNC_IMPL_PROD)
+    target_sources(${COMPONENT_LIB} PRIVATE "func_prod.c")
+endif()

examples/build_system/cmake/multi_config/main/Kconfig.projbuild

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+menu "Example Product Configuration"
+
+    config EXAMPLE_PRODUCT_NAME
+        string "Product name"
+        default "Not set"
+        help
+            Product name used in the example
+
+    choice EXAMPLE_FUNC_IMPL
+        prompt "Implementation of function 'func'"
+        help
+            Select one of the implementations of 'func' to be used in the app.
+            This setting is used in component CMakeLists.txt.
+
+        config EXAMPLE_FUNC_IMPL_DEV
+            bool "Development (func_dev.c)"
+        config EXAMPLE_FUNC_IMPL_PROD
+            bool "Production (func_prod.c)"
+    endchoice
+endmenu

examples/build_system/cmake/multi_config/main/func.h

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+/* Multiple build configurations example
+
+   This example code is in the Public Domain (or CC0 licensed, at your option.)
+
+   Unless required by applicable law or agreed to in writing, this
+   software is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+   CONDITIONS OF ANY KIND, either express or implied.
+*/
+
+#pragma once
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief This function has different implementations depending on the product type.
+ *
+ * See func_dev.c and func_prod.c. Which of the files is compiled is determined in
+ * CMakeLists.txt,
+ */
+void func(void);
+
+#ifdef __cplusplus
+}
+#endif

examples/build_system/cmake/multi_config/main/func_dev.c

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+/* Multiple build configurations example
+
+   This example code is in the Public Domain (or CC0 licensed, at your option.)
+
+   Unless required by applicable law or agreed to in writing, this
+   software is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+   CONDITIONS OF ANY KIND, either express or implied.
+*/
+
+#include <stdio.h>
+#include "func.h"
+
+void func(void)
+{
+    printf("func() from func_dev.c (Development) called.\n");
+}

examples/build_system/cmake/multi_config/main/func_prod.c

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+/* Multiple build configurations example
+
+   This example code is in the Public Domain (or CC0 licensed, at your option.)
+
+   Unless required by applicable law or agreed to in writing, this
+   software is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+   CONDITIONS OF ANY KIND, either express or implied.
+*/
+
+#include <stdio.h>
+#include "func.h"
+
+void func(void)
+{
+    printf("func() from func_prod.c (Production) called.\n");
+}

examples/build_system/cmake/multi_config/main/multi_config_example_main.c

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+/* Multiple build configurations example
+
+   This example code is in the Public Domain (or CC0 licensed, at your option.)
+
+   Unless required by applicable law or agreed to in writing, this
+   software is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+   CONDITIONS OF ANY KIND, either express or implied.
+*/
+#include <stdio.h>
+#include "sdkconfig.h"
+#include "func.h"
+
+void app_main(void)
+{
+    printf("This app is built for running on: " CONFIG_EXAMPLE_PRODUCT_NAME "\n");
+
+    /* This will call func() either from func_dev.c or func_prod.c, depending on
+     * the build configuration.
+     */
+    func();
+
+    printf("See README.md for building and running other app configurations.\n");
+}

examples/build_system/cmake/multi_config/sdkconfig.defaults

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+# In this example, the default build (obtained with 'idf.py build')
+# targets a hypothetical development platform.
+CONFIG_EXAMPLE_PRODUCT_NAME="Blinky Development Board"
+
+# This selects 'func_dev.c' file to be compiled and linked, see CMakeLists.txt.
+CONFIG_EXAMPLE_FUNC_IMPL_DEV=y

examples/build_system/cmake/multi_config/sdkconfig.prod1

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+# This build configuration is for a specific product:
+CONFIG_EXAMPLE_PRODUCT_NAME="Blinky Smart Light"

examples/build_system/cmake/multi_config/sdkconfig.prod2

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+# This build configuration is for a specific product:
+CONFIG_EXAMPLE_PRODUCT_NAME="Blinky Smart Switch"

examples/build_system/cmake/multi_config/sdkconfig.prod_common

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+# Common settings for production builds.
+# For product-specific settings, see sdkconfig.prod1 and sdkconfig.prod2.
+
+# This selects 'func_prod.c' file to be compiled and linked, see CMakeLists.txt.
+CONFIG_EXAMPLE_FUNC_IMPL_PROD=y
+
+# This sdkconfig file can also include additional options specific to this
+# configuration. For example, we can optimize the binary size of a production
+# build, and remove all log messages.
+CONFIG_COMPILER_OPTIMIZATION_SIZE=y
+CONFIG_BOOTLOADER_LOG_LEVEL_NONE=y
+CONFIG_LOG_DEFAULT_LEVEL_NONE=y