reproduce:
        # https://github.com/frank-w/BPI-R2-4.14/commit/6a437b7be07f8f29f1f3f13e027468b07ab75bfd
        git remote add frank-w-bpi-r2-4.14 https://github.com/frank-w/BPI-R2-4.14
        git fetch --no-tags frank-w-bpi-r2-4.14 7.4-rc
        git checkout 6a437b7be07f8f29f1f3f13e027468b07ab75bfd
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # pip install -r Documentation/sphinx/requirements.txt
        make htmldocs