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>

src/soc/intel/baytrail/Makefile.inc

@@ -53,7 +53,7 @@ ramstage-$(CONFIG_ELOG) += elog.c
 # Remove as ramstage gets fleshed out
 ramstage-y += placeholders.c
 
-CPPFLAGS += -Isrc/soc/intel/baytrail/
+CPPFLAGS_common += -Isrc/soc/intel/baytrail/
 
 # Run an intermediate step when producing coreboot.rom
 # that adds additional components to the final firmware