tools: kwbimage: Extract main data image without -p arg for dumpimage

When there is no -p argument for dumpimage tool specified, extract the main
data image from kwbimage file. This makes dumpimage consistent with other
image formats.

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