update readme

Author: Gitshaoxiang

README.md

@@ -8,14 +8,15 @@ 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
+. ./esp-idf/export.sh 
 ```
 
 ### Building the firmware
 ```shell
 git clone https://github.com/m5stack/uiflow_micropython
 cd uiflow_micropython/m5stack
-make littlefs
 make submodules
+make littlefs
 make mpy-cross
 make -j
 make deploy PORT=/dev/ttyUSBx BAUD=1500000