Choice of firmware is pretty irrelevant (as long as it's no bugridden POS...) as you will need to writer proper driver for the HW anyways.
All UBoot,OF,EFI,??? really do is scan PCI/AGP/... for devices, do some basic setup, create a device-tree of some sort and than load some OS image.
Putting the Quark-kernel + ABox into and UBoot compatible file is propraly trivial. Adapting that kernel to traverse a UBoot device-tree instead of an OF one doesn't sound like that big of a task. Writing drivers for all those components on the other hand....
