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>

arch/blackfin/cpu/Makefile

@@ -6,7 +6,7 @@
 # (C) Copyright 2000-2006
 # Wolfgang Denk, DENX Software Engineering, wd@denx.de.
 #
-# Licensed under the GPL-2 or later.
+# SPDX-License-Identifier:	GPL-2.0+
 #
 
 extra-y := init.elf