reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/efi/efi.git/commit/?id=43039cdd1a14ed12c7b506a2d78c7fa708444b06
        git remote add efi https://git.kernel.org/pub/scm/linux/kernel/git/efi/efi.git
        git fetch --no-tags efi next
        git checkout 43039cdd1a14ed12c7b506a2d78c7fa708444b06
        # 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 fs/efivarfs/