build(Makefile): switch-develop target for manual develop switching

Makefile

@@ -404,6 +404,14 @@ new-develop:
 	$(NEW_DEVELOP)
 	mkdir -p $(DEVELOP)
 	$(MAKE) develop/.current
+.PHONY: switch-develop
+# HELP: switch current develop instance to DEVELOP=...
+switch-develop:
+	if ! [ -e develop/$(DEVELOP) ]; then \
+	  echo "Specified develop $(DEVELOP) does not exist! Not switching." ; \
+	  exit 1 ; \
+	fi ; \
+	echo "$(DEVELOP)" > develop/.current
 .PHONY: --develop
 --develop:
 	if ! [[ -e develop ]]; then \