Select factory image when --factory is given

common.sh

Index: common.sh
diff --git a/common.sh b/common.sh
index 09094a8bc82fa9716348a303a8a947df74a08666..18e04cdd9a6ff0ca4718ce1d55fa6256f75b768a 100644
--- a/common.sh
+++ b/common.sh
@@ -645,7 +645,11 @@ prepare_test_image() {
     --image="$1/$2" --noinplace ${args}
 
   # From now on we use the just-created test image
-  CHROMEOS_RETURN_VAL="$1/${CHROMEOS_TEST_IMAGE_NAME}"
+  if [ ${FLAGS_factory} -eq ${FLAGS_TRUE} ]; then
+    CHROMEOS_RETURN_VAL="$1/${CHROMEOS_FACTORY_TEST_IMAGE_NAME}"
+  else
+    CHROMEOS_RETURN_VAL="$1/${CHROMEOS_TEST_IMAGE_NAME}"
+  fi
 }
 
 # Check that the specified file exists.  If the file path is empty or the file