make main() in snow's romstage.c our romstage entry point

Our earlier attempt was jumping straight from asm to the old u-boot
board_init_f in lowlevel_init_c.c. We are getting ready to transition
to using a real bootblock for ARM, so add romstage.c to the files
compiled and we'll make main() our entry point.

This also updates romstage.ld to place main() (*(.text.startup)) at
the beginning of romstage.

Change-Id: Ifc77a6bfba27d915c4cad62c6c8040665294628a
Signed-off-by: David Hendricks <dhendrix@chromium.org>
Reviewed-on: http://review.coreboot.org/2163
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/mainboard/google/snow/Makefile.inc

@@ -17,7 +17,7 @@
 ## Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
 ##
 
-#romstage-y += romstage.c
+romstage-y += romstage.c
 
 # ramstage-y += ec.c