tell IDF build system to use Clang toolchain (IDFGH-14189)

[igrr]

Draft implementation of #8.

The issues with this are:

• if the project was already configured, IDF_TOOLCHAIN=clang environment variable has no effect, since it is cached in CMakeCache.txt. So the project will continue using a GCC toolchain even after idf.py reconfigure
• if the project hasn't been configured yet, IDF_TOOLCHAIN=clang will get recognized and Clang toolchain will be used; however if later the user runs idf.py build, Clang toolchain will also be used for the build. This might be unexpected, since the behavior (the toolchain used) will be different depending on whether the user did run idf.py clang-check or not.

I think the cleaner solution would be to ignore the args.build_dir and always run a CMake configuration stage from scratch in a temporary directory. There is a downside that this will be slower, though.

[samiralavi]

I think you PR addresses my issue as well: #45