Boot from USB Disk - Rockchip Wiki

Same as boot-from-sdcard, but please note that u-boot aren't allowed to boot from usb-disk because of the maskrom's limition, only kernel and rootfs can load from usb-disk.

Difference:

label kernel-4.4
    kernel /zImage
    fdt /rk3288-fennec.dtb
    append  earlyprintk console=ttyS2,115200n8 rw root=/dev/sda1 rootfstype=ext4 init=/sbin/init

gpt write usb 0 $partitions
ums 0 usb 0