As we are sending C continuously, there is a chance that the
sender(remote) receive an C after sending the first handshake package.
So the sender will interpret it as NAK and re-send the package. So we
just ignore it and proceed.
Real-YModem implemented a flexible YModem support. It use callback-based
structure to let the user application to deal with the data. It contains
3 examples:
1. echo.c: write the data recieved on YModem to an other device
2. null.c: discard the YModem data
3. tofile.c: write the data to the file system
Currently, it does not support batch file transmission.
