update the step for build and modify Makefile for init submodule

Author: imliubo

README.md

@@ -1,9 +1,18 @@
 # uiflow_micropython
 
-## build
+## How to build
+### Setting up ESP-IDF and the build environment
+```shell
+mkdir uiflow_workspace && cd uiflow_workspace
+git clone https://github.com/m5stack/esp-idf.git
+git -C esp-idf checkout 014ee65f1f5e291230e398c4913020be9a6278a1
+git -C esp-idf submodule update --init --recursive
+./esp-idf/install.sh
+```
 
+### Building the firmware
 ```shell
-git clone https://github.com/m5stack/uiflow_micropython --recursive
+git clone https://github.com/m5stack/uiflow_micropython
 cd uiflow_micropython/m5stack
 make submodules
 make mpy-cross

m5stack/Makefile

@@ -17,7 +17,7 @@ PYTHON ?= python3
 
 GIT_SUBMODULES = lib/berkeley-db-1.xx
 
-.PHONY: all clean deploy erase submodules FORCE
+.PHONY: all clean deploy erase mpy-cross submodules FORCE
 
 MAKEFILE_DIR:=$(dir $(abspath $(lastword $(MAKEFILE_LIST))))
 
@@ -61,5 +61,9 @@ erase:
 mpy-cross:
 	make -C ../micropython/mpy-cross
 
+# The two submodules do not belong to the same repository
+# micropython submodule belong to uiflow_micropython
+# berkeley-db-1.xx submodule belong to micropython submodule
 submodules:
+	git submodule update --init ../micropython
 	cd ./../micropython	&& git submodule update --init $(addprefix ./,$(GIT_SUBMODULES)) && cd -