reproduce:
        # https://github.com/Freescale/linux-fslc/commit/2526990f17d5ceac745910b95b83973586a6a455
        git remote add freescale-fslc https://github.com/Freescale/linux-fslc
        git fetch --no-tags freescale-fslc 6.6-1.0.x-imx
        git checkout 2526990f17d5ceac745910b95b83973586a6a455
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs