Print the message on QEMU
Not to kill itself
Create a new task to implement shell.
Porting the usart config of rtenv.
Enable user to input cmd to shell.

1.delete the romfs task.

2.modify stdin_read reference to tim37021.
Porting some rtenv function to rtenv_ultil.c.
Porting the shell to freertos with help function.

1.Create two new files: shell.c shell.h

2.Portiong help function.