Reduce number of per-mainboard changes

- Add mainboard_smi.c from arch/x86/Makefile if it's there
- Add mainboard's chromeos.c from the chromeos Makefile

Change-Id: I3f80e2cb368f88d2a38036895a19f3576dd9553b
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/1835
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/vendorcode/google/chromeos/Makefile.inc

@@ -26,3 +26,7 @@ ramstage-y += gnvs.c
 romstage-y += fmap.c
 ramstage-y += fmap.c
 smm-y += fmap.c
+ifneq ($(wildcard src/mainboard/$(MAINBOARDDIR)/chromeos.c),)
+ramstage-srcs += src/mainboard/$(MAINBOARDDIR)/chromeos.c
+romstage-srcs += src/mainboard/$(MAINBOARDDIR)/chromeos.c
+endif