tools: kwboot: Fix usage of -D without -t

When -D is specified then both bootmsg and debugmsg are not set, but
imgpath is set. Fix this check for valid and required parameters.

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