After refactoring the target components (e.g. esp32) no longer contained any real functionality.
What remained in these components have been moved elsewhere and the component itself deleted from the
build system.
This commit updates the visibility of various header files and cleans up
some unnecessary inclusions. Also, this commit removes certain header
include paths which were maintained for backward compatibility.
Also add some explanation about configuring the project in the README
Previously with the default build-esp32.sh script, the build directory was
deleted and recreated so the project would always be built with default config.
GCC prefixes underscore on symbol names on macOS, and so the set
undefined symbol `-u app_main` won't work. However, this is unecessary,
as the source file that defines it is specified directly to the
executable, and not through a static library.
Closes https://github.com/espressif/esp-idf/issues/4168
