build: avoid failing in append-metadata if image could not be generated

The image build might have failed due to a size check

Signed-off-by: Felix Fietkau <[email protected]>

include/image-commands.mk

@@ -281,7 +281,7 @@ metadata_json = \
 	}'
 
 define Build/append-metadata
-	$(if $(SUPPORTED_DEVICES),echo $(call metadata_json,$(SUPPORTED_DEVICES)) | fwtool -I - $@)
+	$(if $(SUPPORTED_DEVICES),-echo $(call metadata_json,$(SUPPORTED_DEVICES)) | fwtool -I - $@)
 endef
 
 define Build/kernel2minor