It defaults to boot off internal 16GB MMC memory which is pre-installed with Ubuntu 18.04 LTS. I have been trying to boot FreeBSD, OpenBSD, and NetBSD on a USB Stick with EFI but it crashes every time. I have seen some reports that this older version of U-boot that is installed had trouble with EFI booting. There is a usbboot command that does not expect EFI and a bootefi command.
