Makefile: add a helper

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>

Makefile

@@ -0,0 +1,27 @@
+# Makefile for rtthread-lab
+
+BSP ?= qemu-vexpress-a9
+BSP_WORKDIR = rt-thread/bsp/$(BSP)/
+BOOT_CMD = qemu-nographic.sh
+
+ifeq ($(G),1)
+  BOOT_CMD := qemu.sh
+endif
+
+help:
+	@echo "Usage:"
+	@echo
+	@echo "init  -- download or update rt-thread"
+	@echo "build -- compile rt-thread"
+	@echo "boot  -- boot rt-thread on qemu"
+	@echo
+init:
+	git submodule update --init --remote .
+
+build:
+	cd $(BSP_WORKDIR)
+	scons
+
+boot:
+	cd $(BSP_WORKDIR)
+	bash $(BOOT_CMD)