reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/ardb/linux.git/commit/?id=b6f238d2e8720acf63467aa0d48d7fc4e31c07c2
        git remote add ardb https://git.kernel.org/pub/scm/linux/kernel/git/ardb/linux.git
        git fetch --no-tags ardb x86-efi-zboot
        git checkout b6f238d2e8720acf63467aa0d48d7fc4e31c07c2

        # 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 drivers/firmware/efi/libstub/