kwbimage: Update help message about how to extract from an existing image

Extracting is now supported by dumpimage, so mention it in help instead
of `kwbimage -x`.

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