Docs: Serialize make examples and make images

Makefile

@@ -979,10 +979,8 @@ docs/source/cmd/abc.rst: $(TARGETS) $(EXTRA_TARGETS)
 	./$(PROGRAM_PREFIX)yosys -p 'help -write-rst-command-reference-manual'
 
 PHONY: docs/gen_examples docs/gen_images docs/guidelines docs/usage docs/reqs
-docs/gen_examples: $(TARGETS)
+docs/gen_examples docs/gen_images: $(TARGETS)
 	$(Q) $(MAKE) -C docs examples
-
-docs/gen_images: $(TARGETS)
 	$(Q) $(MAKE) -C docs images
 
 DOCS_GUIDELINE_FILES := GettingStarted CodingStyle