8331939: Add custom hook for TestImage

Reviewed-by: mikael

Author: erikj79

make/TestImage.gmk

@@ -28,6 +28,9 @@ default: all
 include $(SPEC)
 include MakeBase.gmk
 
+# Hook to include the corresponding custom file, if present.
+$(eval $(call IncludeCustomExtension, TestImage-pre.gmk))
+
 ############################################################################
 
 BUILD_INFO_PROPERTIES := $(TEST_IMAGE_DIR)/build-info.properties