The command available I'm able to use is bootm from the u-boot console, here a short help text on it. In my case I load the initramfs using a tftp server (and u-boot acts as a tftp client) to the device first.
In the Booting an initramfs image it's explained how to boot the recovery image.
I'm not sure if you could also use the USB port/USB stick to load the initramfs instead of TFTP.