tools: kwboot: Allow to mix positional arguments with option -b

Commit 9e6d71d2 ("tools: kwboot: Allow to use -b without image path as
the last getopt() option") broke usage of kwboot with following arguments:

  kwboot -t -B 115200 /dev/ttyUSB0 -b u-boot-spl.kwb

Fix parsing of option -b with optional argument again.

Fixes: 9e6d71d2 ("tools: kwboot: Allow to use -b without image path as the last getopt() option")
Signed-off-by: Pali Rohár <pali@kernel.org>
Reported-by: Tony Dinh <mibodhi@gmail.com>
Tested-by: Tony Dinh <mibodhi at gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>