reproduce:
        # https://github.com/Freescale/linux-fslc/commit/d911e8afd451766851d6dd41dc184efdffacde9a
        git remote add freescale-fslc https://github.com/Freescale/linux-fslc
        git fetch --no-tags freescale-fslc 6.1-2.2.x-imx
        git checkout d911e8afd451766851d6dd41dc184efdffacde9a
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs