build: separate CPPFLAGS from CFLAGS

There are a couple of places where CPPFLAGS are
pasted into CFLAGS, eliminate them.

Change-Id: Ic7f568cf87a7d9c5c52e2942032a867161036bd7
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/5765
Tested-by: build bot (Jenkins)
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>

Makefile

@@ -242,7 +242,7 @@ ifn$(EMPTY)def $(1)-objs_$(2)_template
 de$(EMPTY)fine $(1)-objs_$(2)_template
 $(obj)/$$(1).$(1).o: src/$$(1).$(2) $(obj)/config.h $(4)
 	@printf "    CC         $$$$(subst $$$$(obj)/,,$$$$(@))\n"
-	$(CC_$(1)) $(3) -MMD $$$$(CFLAGS_$(1)) -c -o $$$$@ $$$$<
+	$(CC_$(1)) $(3) -MMD $$$$(CPPFLAGS_$(1)) $$$$(CFLAGS_$(1)) -c -o $$$$@ $$$$<
 en$(EMPTY)def
 end$(EMPTY)if
 endef