tools: kwbimage: Fix mkimage/dumpimage -l argument

Do not check for kwbimage configuration file when just showing information
about existing kwbimage file.

The check for kwbimage configuration file is required only when creating
kwbimage, not when showing information about image or when extracting data
from image.

With this change, it is possible to call mkimage -l and dumpimage -l also
for existing kwbimage file.

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