This feature was added when we thought that target components might be
added out of IDF tree. That never became possible, and with recent
changes in IDF architecture it is unlikely that this feature will be
necessary any time soon. Remove it, simplifying the code.
This is an experimental feature intended at the moment for ESP-IDF
developers only.
If IDF_TOOLCHAIN=clang environment variable or CMake variable is set,
use toolchain-clang-esp32xx.cmake instead of toolchain-esp32xx.cmake.
These changes aren't sufficient to actually build any IDF project with
clang; subsequent commits add a few workarounds required to do this.
Toolchain files are added for esp32 and esp32s2, which are the targets
supported in our llvm-project fork at the moment.
