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>

doc/DocBook/Makefile

@@ -5,6 +5,9 @@
 # to document the SRC - and how to read it.
 # To add a new book the only step required is to add the book to the
 # list of DOCBOOKS.
+#
+# SPDX-License-Identifier:	GPL-2.0
+#
 
 DOCBOOKS := linker_lists.xml stdio.xml