Finally, a somewhat related question for a wider audience: For obvious reasons I wanted to change to Memtest as my payload. As it turns out, the size of that is almost 100 kB, which is bigger than the 64kB available (256 kB flashchip, normal and fallback images with each 64 kB for coreboot and 64 kb for payload). I'm going to look if it is possible to make an image with only normal or fallback, and give the extra space to the payload. If anyone has some pointers for me, they would be most appreciated.
Fallback only isn't too hard to do. Look at any of the Config-lab.lb files for boards that are supported in buildrom.
Thanks, Myles