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.
I believe these apply to the OS load stage. The open question is how to affect u-boot’s view of the world, in terms of eth ports (keeping them disabled). I would guess this would have to do with the actual u-boot image (perhaps a dtb which is linked into the u-boot image?).