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>
diff --git a/src/mainboard/lenovo/x60/Makefile.inc b/src/mainboard/lenovo/x60/Makefile.inc
index 83cdeb9..0b01fe4 100644
--- a/src/mainboard/lenovo/x60/Makefile.inc
+++ b/src/mainboard/lenovo/x60/Makefile.inc
@@ -17,6 +17,6 @@
 ## Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
 ##
 
-smm-$(CONFIG_HAVE_SMI_HANDLER) += mainboard_smi.c dock.c
+smm-$(CONFIG_HAVE_SMI_HANDLER) += dock.c
 romstage-y += dock.c
 ramstage-y += dock.c