build: CPPFLAGS is more common than INCLUDES

Rename INCLUDES to CPPFLAGS since the latter is more
commonly used for preprocessor options.

Change-Id: I522bb01c44856d0eccf221fa43d2d644bdf01d69
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/5764
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
 
-INCLUDES += -Isrc/soc/intel/baytrail/
+CPPFLAGS += -Isrc/soc/intel/baytrail/
 
 # Run an intermediate step when producing coreboot.rom
 # that adds additional components to the final firmware