tools: kwbimage: Do not show mkimage error message in dumpimage

When pflag is set then kwbimage was invoked by dumpimage and not mkimage.
So do not show mkimage error message in this case.

Signed-off-by: Pali Rohár <pali@kernel.org>
Tested-by: Tony Dinh <mibodhi@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>