Treewide: Fix incorrect SPDX license strings

These strings didn't match the license names exactly, so update them
to match.

Change-Id: Ib946eb15ca5fa64cbd6b657350b989b4a4c1b7b7
Signed-off-by: Martin Roth <gaumless@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/80583
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>

util/kconfig/expr.c

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL-2.0
+// SPDX-License-Identifier: GPL-2.0-only
 /*
  * Copyright (C) 2002 Roman Zippel <zippel@linux-m68k.org>
  */