Move USART2 test function to a new folder.
User can change to directory "peripheral_test" and do the test
function.
Delete some useless USART code.
Start porting serial_io.c
Port main.c from branch "master". Not finished yet.
Warning: these code may have some issue. I have not done test.
Change inlucde lib headers to CM4F version
Enable Makefile compile all of my source code.
This commit has one issue. vTaskList might not a function in
FreeRTOS V7.5.3.
Port my shell enviroment finished. Not test yet.
In this commit, I have done basic shell with linenoise.
But semi-host and ps cannot work properly.