[BUILD] Ignore 'docker-clean' Output on Failure

When docker images don't exist, or have already been removed, the
'docker-clean' target lists failures for 'docker rmi' because it
requires at least on argument.

This ensures at least one argument is passed ('' if no images).

Change-Id: Ie2a47ca028dbafd65fd643c0272bf64fc7bdd030
Signed-off-by: Trevor Bramwell <tbramwell@linuxfoundation.org>

Makefile

@@ -215,7 +215,7 @@ protos: gotools
 
 %-docker-clean:
 	$(eval TARGET = ${patsubst %-docker-clean,%,${@}})
-	-docker images -q $(PROJECT_NAME)-$(TARGET) | xargs docker rmi -f
+	-docker images -q $(PROJECT_NAME)-$(TARGET) | xargs -I '{}' docker rmi -f '{}'
 	-@rm -rf build/image/$(TARGET) ||:
 
 docker-clean: $(patsubst %,%-docker-clean, $(IMAGES))