Boot Raspberry Pi 3B+ from usb instead of microSD?

[spiritomb]

I followed the instructions for installing the pi 3 image on a microSD card according to the wiki, and everything works well. But I haven't been able to figure out how to set up u-boot to have the root filesystem on a usb device instead of the microSD card.

Previously on this pi, I used the armv7 image and put the root fs on a usb device, altering /boot/cmdline.txt and /etc/fstab accordingly without issue. But the pi 3 image lacks the cmdline.txt file. Is it possible to put the boot partition on the microSD card and the root partition on a usb device with the pi 3 image? If so, how do I set that up?