[config] Dist supports custom super image script

Some boards need to supply a custom super image script, and this
is supported via the BOARD_CUSTOM_BUILD_SUPER_IMAGE variable.

However, this variable is not used by img_from_target_files.py.

Change img_from_target_files.py to call the supplied script.

Bug: None
Change-Id: I6bca937534f58459d3419139ee7a37a10138ab8f
diff --git a/core/Makefile b/core/Makefile
index 07d705e..6f40063 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -7118,6 +7118,7 @@
 	PATH=$(INTERNAL_USERIMAGES_BINARY_PATHS):$(dir $(ZIP2ZIP)):$$PATH \
 	    $(IMG_FROM_TARGET_FILES) \
 	        --additional IMAGES/VerifiedBootParams.textproto:VerifiedBootParams.textproto \
+	        --build_super_image $(BUILD_SUPER_IMAGE) \
 	        $(BUILT_TARGET_FILES_PACKAGE) $@
 
 $(call declare-1p-container,$(INTERNAL_UPDATE_PACKAGE_TARGET),)