For me, trying to get ubuntu 16.04 to boot, the ext4ls usb 0:1 boot returns the content of bin/ directory.
obviously the boot fails to find the files :
Marvell>> run bootusb
(Re)start USB...
USB0: Register 2000104 NbrPorts 2
Starting the controller
USB XHCI 1.00
scanning bus 0 for devices... 2 USB Device(s) found
scanning usb for storage devices... 1 Storage Device(s) found
scanning usb for ethernet devices... 0 Ethernet Device(s) found
** File not found boot/Image **
** File not found boot/armada-3720-community.dtb **
Bad Linux ARM64 Image magic!