Avoid replacement of existing configuration file

Makefile

@@ -16,11 +16,11 @@ firewall: $(INSTALL_DIR)/lfirewall config $(SYSTEMD_SERVICE_FILE)
 
 config: 
 	$(MKDIR) -p $(FIREWALL_ETC_DIR)
-	$(CP) etc/$(FIREWALL_CONF) $(FIREWALL_ETC_DIR)/
-	
+	@if [ ! -f $(FIREWALL_ETC_DIR)/$(FIREWALL_CONF) ] ; then $(CP) etc/$(FIREWALL_CONF) $(FIREWALL_ETC_DIR)/ ; else echo "The file $(FIREWALL_ETC_DIR)/$(FIREWALL_CONF) already exists" ; fi
+
 $(INSTALL_DIR)/lfirewall: scripts/lfirewall
 	$(CP) $< $@
 
 $(SYSTEMD_SERVICE_FILE): systemd/$(SERVICE_FILE)
 	$(CP) $< $@
-	
+