从来没用过这块板子，是真的不会，时间又紧, 又找不到教程，
要求是
This task is to develop a program to enter a loop that receives characters from the UART and then writes
it back to the UART. This is known as “echoing” back. A terminal emulator running on the PC will be used
to send characters to the board and display what the board sends back.
Requirement and Configuration
Set the baud rate to 115200, no parity check, one start bit, one stop bit.
找一下板子的串口收发例程，弄清楚怎么收发，然后把收到的数据原样发回去就是了