payloads: Rename Makefiles from .inc to .mk

The .inc suffix is confusing to various tools as it's not specific to
Makefiles. This means that editors don't recognize the files, and don't
open them with highlighting and any other specific editor functionality.

This issue is also seen in the release notes generation script where
Makefiles get renamed before running cloc.

Signed-off-by: Martin Roth <gaumless@gmail.com>
Change-Id: Ie7038712de8cc646632d5e7d29550e3260bf2c62
Reviewed-on: https://review.coreboot.org/c/coreboot/+/80103
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Reviewed-by: Maximilian Brune <maximilian.brune@9elements.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

payloads/libpayload/gdb/Makefile.mk

@@ -0,0 +1,16 @@
+##
+## Copyright 2014 Google 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.
+##
+## This program is distributed in the hope that it will be useful,
+## but without any warranty; without even the implied warranty of
+## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+## GNU General Public License for more details.
+
+libgdb-y += commands.c
+libgdb-y += stub.c
+libgdb-y += transport.c