reproduce (this is a W=1 build):
        # https://github.com/frank-w/BPI-R2-4.14/commit/ce8c582c6121469db64197b1b4aa4e4c377c660f
        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 pr/50
        git checkout ce8c582c6121469db64197b1b4aa4e4c377c660f
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=um olddefconfig
        make W=1 O=build_dir ARCH=um SHELL=/bin/bash