Update Makefile as per the tutorial, enhance it

examples/stm32/stm32-nucleo-f746z/Makefile

@@ -1,9 +1,17 @@
 NAME ?= $(notdir $(CURDIR))
 
-build:
+$(NAME):
 	git clone --depth 1 https://github.com/mongoose-examples/$(NAME)
+
+build: | $(NAME)
 	cp ./../../../mongoose.[ch] $(NAME)/Middlewares/Third_Party/Mongoose/
 	make -C $(NAME) build
 
 clean:
 	rm -rf $(NAME)
+
+flash:
+	make -C $(NAME) flash
+
+.PHONY: flash
+