I have never used Uboot, but from what I read it is used to boot a server to a Linux kernel or some sort of PXE boot type function. This is not going to work for a switch or a router. You are going to need to SSH, web, or console into the device and run the commands to perform an upgrade. Please look at the Upgrade Guide on the Ruckus Support page for more information.
Sorry, I completely misunderstood the original question. You should be able to run show_image to see what image is in primary and secondary. Then you can run boot_primary or boot_secondary to boot that image. If there are no images in the primary or secondary, you can run tftpboot (with parameters) to get an image from a tftp server.