tools: kwbimage: Change maximum number of arguments in binary header to 256

The number is stored in one byte, so the maximum should be 255.

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