reproduce (this is a W=1 build):
        # https://github.com/multipath-tcp/mptcp_net-next/commit/2426143e89ea7b4900ad9a07ecd7fee2cb3781e6
        git remote add mptcp https://github.com/multipath-tcp/mptcp_net-next.git
        git fetch --no-tags mptcp export
        git checkout 2426143e89ea7b4900ad9a07ecd7fee2cb3781e6

        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=i386 olddefconfig
        make W=1 O=build_dir ARCH=i386 SHELL=/bin/bash