docs: add style guide

This adds initial code style guide. Only section on code formatting is written, other sections to be added later.
Also adds scripts to format code using astyle.

docs/style-guide.rst

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+Espressif IoT Development Framework Style Guide
+===============================================
+
+
+About this guide
+~~~~~~~~~~~~~~~~
+
+Purpose of this style guide is to encourage use of common coding practices within the ESP-IDF. 
+
+Style guide is a set of rules which are aimed to help create readable, maintainable, and robust code. By writing code which looks the same way across the code base we help others read and comprehend the code. By using same conventions for spaces and newlines we reduce chances that future changes will produce huge unreadable diffs. By following common patterns for module structure and by using language features consistently we help others understand code behavior.
+
+We try to keep rules simple enough, which means that they can not cover all potential cases. In some cases one has to bend these simple rules to achieve readability, maintainability, or robustness.
+
+When doing modifications to third-party code used in ESP-IDF, follow the way that particular project is written. That will help propose useful changes for merging into upstream project. 
+
+C code formatting
+~~~~~~~~~~~~~~~~~
+
+Indentation
+-----------
+
+Use 4 spaces for each indentation level. Don't use tabs for indentation. Configure the editor to emit 4 spaces each time you press tab key.
+
+Vertical space
+--------------
+
+Place one empty line between functions. Don't begin or end a function with an empty line.
+::
+
+    void function1()
+    {
+        do_one_thing();
+        do_another_thing();
+                                    // INCORRECT, don't place empty line here
+    }
+                                    // place empty line here
+    void function2()
+    {
+                                    // INCORRECT, don't use an empty line here
+        int var = 0;
+        while (var < SOME_CONSTANT) {
+            do_stuff(&var);
+        }
+    }
+
+Horizontal space
+----------------
+
+Always add single space after conditional and loop keywords::
+    
+    if (condition) {    // correct
+        // ...
+    }
+
+    switch (n) {        // correct
+        case 0:
+            // ...
+    }
+
+    for(int i = 0; i < CONST; ++i) {    // INCORRECT
+        // ... 
+    }
+
+Add single space around binary operators. No space is necessary for unary operators. It is okay to drop space around multiply and divide operators::
+    
+    const int y = y0 + (x - x0) * (y1 - y0) / (x1 - x0);    // correct
+
+    const int y = y0 + (x - x0)*(y1 - y0)/(x1 - x0);        // also okay
+
+    int y_cur = -y;                                         // correct
+    ++y_cur;
+
+    const int y = y0+(x-x0)*(y1-y0)/(x1-x0);                // INCORRECT
+
+
+No space is necessary around ``.`` and ``->`` operators.
+
+
+Sometimes adding horizontal space within a line can help make code more readable. For example, you can add space to align function arguments::
+
+    gpio_matrix_in(PIN_CAM_D6,   I2S0I_DATA_IN14_IDX, false);
+    gpio_matrix_in(PIN_CAM_D7,   I2S0I_DATA_IN15_IDX, false);
+    gpio_matrix_in(PIN_CAM_HREF, I2S0I_H_ENABLE_IDX,  false);
+    gpio_matrix_in(PIN_CAM_PCLK, I2S0I_DATA_IN15_IDX, false);
+
+Note however that if someone goes to add new line with a longer identifier as first argument (e.g.  ``PIN_CAM_VSYNC``), it will not fit. So other lines would have to be realigned, adding meaningless changes to the commit. 
+
+Therefore, use horizontal alignment sparingly, especially if you expect new lines to be added to the list later.
+
+Never use TAB characters for horizontal alignment.
+
+Never add trailing whitespace at the end of the line.
+
+
+Braces
+------
+
+- Function definition should have a brace on a separate line::
+
+    // This is correct:
+    void function(int arg)
+    {
+
+    }
+
+    // NOT like this:
+    void function(int arg) {
+
+    }
+
+- Within a function, place opening brace on the same line with conditional and loop statements::
+    
+    if (condition) {
+        do_one();
+    }
+    else if (other_condition) {
+        do_two();
+    }
+
+
+Comments
+--------
+
+Use ``//`` for single line comments. For multi-line comments it is okay to use either ``//`` on each line or a ``/* */`` block.
+
+Although not directly related to formatting, here are a few notes about using comments effectively.
+
+- Don't use single comments to disable some functionality::
+
+    void init_something()
+    {
+        setup_dma();
+        // load_resources();                // WHY is this thing commented, asks the reader?
+        start_timer();
+    }
+
+- If some code is no longer required, remove it completely. If you need it you can always look it up in git history of this file. If you disable some call because of temporary reasons, with an intention to restore it in the future, add explanation on the adjacent line::
+
+    void init_something()
+    {
+        setup_dma();
+        // TODO: we should load resources here, but loader is not fully integrated yet.
+        // load_resources();
+        start_timer();
+    }
+
+- Same goes for ``#if 0 ... #endif`` blocks. Remove code block completely if it is not used. Otherwise, add comment explaining why the block is disabled. Don't use ``#if 0 ... #endif`` or comments to store code snippets which you may need in the future.
+
+- Don't add trivial comments about authorship and change date. You can always look up who modified any given line using git. E.g. this comment adds clutter to the code without adding any useful information::
+
+    void init_something()
+    {
+        setup_dma();
+        // XXX add 2016-09-01
+        init_dma_list();
+        fill_dma_item(0);
+        // end XXX add
+        start_timer();
+    }
+
+
+Formatting your code
+--------------------
+
+You can use ``astyle`` program to format your code according to the above recommendations.
+
+If you are writing a file from scratch, or doing a complete rewrite, feel free to re-format the entire file. If you are changing a small portion of file, don't re-format the code you didn't change. This will help others when they review your changes.
+
+To re-format a file, run::
+
+    tools/format.sh components/my_component/file.c
+
+Structure and naming
+~~~~~~~~~~~~~~~~~~~~
+
+To be written.
+
+Language features
+~~~~~~~~~~~~~~~~~
+
+To be written.
+

tools/format-minimal.sh

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+#!/bin/bash
+# Runs astyle with parameters which should be checked in a pre-commit hook
+astyle \
+	--style=otbs \
+	--indent=spaces=4 \
+	--convert-tabs \
+	--keep-one-line-statements \
+	--pad-header \
+	"$@"

tools/format.sh

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+#!/bin/bash
+# Runs astyle with the full set of formatting options
+astyle \
+	--style=otbs \
+	--indent=spaces=4 \
+	--convert-tabs \
+	--align-pointer=name \
+	--align-reference=name \
+	--keep-one-line-statements \
+	--pad-header \
+	--pad-oper \
+	"$@"