uboot-imx

rockchip: mkimage: play nice with dumpimage

Dumpimage (it invoked with "-T rkspi" or "-T rksd") would not work due
to check_params failing. These changes ensure that we can both be called
with an empty imagename.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Simon Glass <sjg@chromium.org>