Merge pull request #2446 from Pinata-Consulting/makefile-do-grt

makefile: add do-grt target, congruent with gui_grt

Author: maliberty

flow/Makefile

@@ -797,6 +797,10 @@ $(eval $(call do-copy,5_route,4_cts.sdc,,.sdc))
 do-route:
 	$(UNSET_AND_MAKE) do-5_1_grt do-5_2_route do-5_3_fillcell do-5_route do-5_route.sdc
 
+.PHONY: do-grt
+do-grt:
+	$(UNSET_AND_MAKE) do-5_1_grt
+
 .PHONY: clean_route
 clean_route:
 	rm -rf output*/ results*.out.dmp layer_*.mps