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>

scripts/Makefile

@@ -3,6 +3,9 @@
 # the kernel for the build process.
 # ---------------------------------------------------------------------------
 # docproc:       Used in Documentation/DocBook
+#
+# SPDX-License-Identifier:	GPL-2.0
+#
 
 # The following hostprogs-y programs are only build on demand
 hostprogs-y += docproc