tools: kwboot: Allow to use option -b without image path

Allow option -b without image path parameter, to send boot pattern and
wait for response but not send any image. This allows to use kwboot just
for processing boot pattern and user can use any other xmodem tool for
transferring the image itself (e.g. sx). Useful for debugging purposes.

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