Currently it is possible to call "kwboot -b -t /dev/ttyUSB0" but not to
call "kwboot -b /dev/ttyUSB0".

Fix it by not trying to process the last argv[], which is non-getopt()
option (tty path) as the image path for -b.

Fixes: c513fe47 ("tools: kwboot: Allow to use option -b without image path")
Reported-by: NMarcel Ziswiler <marcel@ziswiler.com>
Signed-off-by: NPali Rohár <pali@kernel.org>
Reviewed-by: NMarcel Ziswiler <marcel@ziswiler.com>
Tested-by: NMarcel Ziswiler <marcel@ziswiler.com>