util: Add SPDX header, replacing boiler plate where applicable

In a few cases a license was added: Stuff coming from Linux is
"GPL-2.0" (not GPL-2.0-only!), build-release is by me and got the
usual GPL-2.0-only treatment. uio_usbdebug and spkmodem had their
licenses propagate to all their files.

Change-Id: Ia5712bbaa417cb9e937834512351fcc0acfa16be
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41202
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

util/scripts/get_maintainer.pl

@@ -8,7 +8,7 @@
 # usage: perl scripts/get_maintainer.pl [OPTIONS] <patch>
 #        perl scripts/get_maintainer.pl [OPTIONS] -f <file>
 #
-# Licensed under the terms of the GNU GPL License version 2
+# SPDX-License-Identifier: GPL-2.0-only
 
 use strict;
 use warnings;