Do CAR variable migration only once

Non-S3 resume paths of sandy/ivybridge call cbmem_initialize()
more than once. Doing car_migrate_variables() more than twice caused
at least loss of some lines in CBMEM console.

Change-Id: Idd14aba9384984aa3a7d38937a4b3572aa5dc088
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/3512
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@google.com>

src/cpu/x86/car.c

@@ -80,6 +80,10 @@ void car_migrate_variables(void)
 	car_migration_func_t *migrate_func;
 	size_t car_data_size = &_car_data_end[0] - &_car_data_start[0];
 
+	/* Check if already migrated. */
+	if (car_migrated)
+		return;
+
 	migrated_base = cbmem_add(CBMEM_ID_CAR_GLOBALS, car_data_size);
 
 	if (migrated_base == NULL) {