rockchip: mkimage: fix mkimage -l for header v1

There are two paths to reach this function, either through mkimage -l or
through dumpimage -l. The latter passes a NULL imagename while the
former passes an empty string. Therefore, let's make both tools behave
the same by handling the empty string the same way as for NULL.

Without this, the only way to get some information out of mkimage -l is
to provide "-n rk3399" for example, which isn't documented in the usage
of the tool.

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>