grub.cfg: add spdx header

it has always been gpl 3 or later, but it helps to have
the license declaration within the file

there's a copying file anyway. put spdx in the config

Signed-off-by: Leah Rowe <leah@libreboot.org>

config/grub/config/grub.cfg

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+
 set prefix=(memdisk)/boot/grub
 
 insmod at_keyboard

config/grub/config/grub_memdisk.cfg

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+
 set prefix=(memdisk)/boot/grub
 
 if [ -f (cbfsdisk)/grub.cfg ]; then