tools: kwboot: Do not change received character in kwboot_xm_recv_reply()

Marvell BootROM expects retransmission of previous xmodem packet only in
the case when it sends NAK response to the host.

Do not change non-xmodem response (possibly UART transfer error) to NAK
in kwboot_xm_recv_reply() function. Allow caller to receive original
response from device.

Change argument 'nak_on_non_xm' to 'stop_on_non_xm'. Instead of changing
non-xmodem character to NAK, stop processing on invalid character and
return it.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>