reproduce (this is a W=1 build):
        # https://github.com/frank-w/BPI-R2-4.14/commit/cb6612a57bbcc77b70a9fc1c521c14c48303714d
        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 6.13-rsslro
        git checkout cb6612a57bbcc77b70a9fc1c521c14c48303714d
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=x86_64 olddefconfig
        make W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash drivers/mmc/host/