mb/google/sarien to zork: 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: Ib8a2ae26ed4380592d15e1a7b2d682639413af01
Reviewed-on: https://review.coreboot.org/c/coreboot/+/80109
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Reviewed-by: Maximilian Brune <maximilian.brune@9elements.com>

src/mainboard/google/skyrim/Makefile.mk

@@ -25,7 +25,7 @@ APCB_PATH=$(src)/mainboard/$(MAINBOARDDIR)
 ifneq ($(wildcard $(APCB_PATH)/$(APCB_NAME)),)
 $(info APCB sources present.)
 
-ifneq ($(wildcard $(src)/mainboard/$(MAINBOARDDIR)/variants/$(VARIANT_DIR)/memory/Makefile.inc),)
+ifneq ($(wildcard $(src)/mainboard/$(MAINBOARDDIR)/variants/$(VARIANT_DIR)/memory/Makefile.mk),)
 
 LIB_SPD_DEPS = $(SPD_SOURCES)