@pacman – I think you tell u-boot where to find this in the very initial setup described here: http://wiki.espressobin.net/tiki-index.php?page=Boot+from+removable+storage+-+Ubuntu#Setting_U-Boot_parameters_2
Specifically when you run:
Marvell>> setenv image_name boot/Image
Marvell>> setenv fdt_name boot/armada-3720-community.dtb
I am running off a microSD card.