reproduce (this is a W=1 build):
        # https://github.com/multipath-tcp/mptcp_net-next/commit/8705d5d473b289fab5a0af20cba9660ca4c6414a
        git remote add mptcp https://github.com/multipath-tcp/mptcp_net-next.git
        git fetch --no-tags mptcp export-net
        git checkout 8705d5d473b289fab5a0af20cba9660ca4c6414a

        # 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