Hi,
according the first problem I've already created a ticket. I (and others in IRC) failed building rom-images using nconfig+make with other payloads than seabios. See the txt-files in the ticket system; make reports something like: "Rule for filo unkown".
The second problem is that a self-build seabios doesn't boot in qemu (again for me and people in IRC). While the default-qemu-bios boots any diskimage just fine, -boot build/coreboot.rom causes seabios to start fine (at first), too. But when it actually tries to boot from an image, the errors: "Graphic initalization failed. Error setting up gfxboot. boot: Could not allocate memory."
If I build a new virtual harddrive, install Ubuntu there (using qemu's default bios) then I'm able to start the new image using my self-build bios.