armv7: unify stage hand-off routines

This replaces the current stage-specific exit/entry functions with
generic versions. Now all stages compile with stage_entry(), which
is placed at .text.stage_entry.armv7, and stage_exit().

Snow's ramstage files are also updated to avoid build breakage.

Change-Id: I953a2c4b8121bd4b66c3362557997a9ca3aa53b0
Signed-off-by: David Hendricks <dhendrix@chromium.org>
Reviewed-on: http://review.coreboot.org/2254
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
Tested-by: build bot (Jenkins)

src/mainboard/google/snow/mainboard.c

@@ -18,6 +18,7 @@
  */
 
 #include <device/device.h>
+#include <console/console.h>
 
 // mainboard_enable is executed as first thing after
 // enumerate_buses().
@@ -31,4 +32,3 @@ struct chip_operations mainboard_ops = {
 	.name	= "Samsung/Google ARM ChromeBook",
 	.enable_dev = mainboard_enable,
 };
-