Add phony firrtl target to just build firrtl file (#317)

common.mk

@@ -43,6 +43,9 @@ $(FIRRTL_FILE) $(ANNO_FILE): generator_temp
 generator_temp: $(SCALA_SOURCES) $(sim_files)
 	mkdir -p $(build_dir)
 	cd $(base_dir) && $(SBT) "project $(SBT_PROJECT)" "runMain $(GENERATOR_PACKAGE).Generator $(build_dir) $(MODEL_PACKAGE) $(MODEL) $(CONFIG_PACKAGE) $(CONFIG)"
+	
+.PHONY: firrtl
+firrtl: $(FIRRTL_FILE)
 
 #########################################################################################
 # create verilog files rules and variables