Various Makefiles: Add SPDX-License-Identifier tags

After consulting with some of the SPDX team, the conclusion is that
Makefiles are worth adding SPDX-License-Identifier tags too, and most of
ours have one.  This adds tags to ones that lack them and converts a few
that had full (or in one case, very partial) license blobs into the
equivalent tag.

Cc: Kate Stewart <kstewart@linuxfoundation.org>
Signed-off-by: Tom Rini <trini@konsulko.com>

board/gdsys/p1022/Makefile

@@ -1,10 +1,7 @@
 #
 # Copyright 2010 Freescale Semiconductor, Inc.
 #
-# This program is free software; you can redistribute it and/or modify it
-# under the terms of the GNU General Public License as published by the Free
-# Software Foundation; either version 2 of the License, or (at your option)
-# any later version.
+# SPDX-License-Identifier:	GPL-2.0+
 #
 
 obj-y	+= law.o