通过TFTP协议从网口下载数据。源码中board_eth_rcv()用于接收数据，将该数据上传给ip层。board_eth_send()用于将上传过来的数据包发送出去。-Through the TFTP protocol to download data from the net mouth. Source board_eth_rcv () used to receive data, the data upload to the ip layer. board_eth_send () is us