Apparently it is possible for U-Boot to boot from USB media like a flash stick. I have no idea how to set up such a thing (yet anyway), but it could be an interesting alternative to using SD cards.
If you type "help" to the U-Boot prompt, you will see:
usb - USB sub-system usbboot - boot from USB deviceYou can then type "usb tree" to see:
USB device tree: 1 Hub (480 Mb/s, 0mA) u-boot EHCI Host Controller 1 Hub (480 Mb/s, 0mA) u-boot EHCI Host ControllerThe first of these is the USB-2.0 host port that is "stood on edge".
There is also the USB 3.0 "OTG" port with a USB-C connector. U-Boot does not know anything about it either. In particular, U-Boot has no support for making the RK3399 a device that can download boot images via a cable to this port.
Tom's electronics pages / tom@mmto.org