ymodem: fix a race-condition in handshaking

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.
Merge pull request #401 from grissiom/fix-ymodem

ymodem: fix a race-condition in handshaking