I compiled the tip version of coreboot following the instructions (http://pcengines.info/forums/?page=post&id=CAA8403D-7135-4EA1-8C7E-41C8B15C6246). I went with the tip of master instead of that specific Gerrit review, since I saw that it had been submitted already. I also went with the branch 'coreboot-4.0.x' of the SeaBIOS mirror, since that's the default branch and the one mentioned in the instructions looked quite old. I did not add iPXE, memtest, or the bootorder thing because I did not think that I needed them.
It sort-of worked in that it started up, but it could not read from the SD card or external USB ports and pressing F10 did nothing during the boot-up. I thought I had bricked it and was about to buy the unbricking thing when I decided to try to remove the internal hard drive (which is blank) and try to boot from the internal USB port (which I understand to be 2.0 instead of the 3.0 external ones?); fortunately, that worked and I was able to reflash the official firmware and all is well again.
Did I pick the wrong branch of SeaBIOS or do something else wrong? I'm interested in building all of this stuff from source for educational reasons.