I chatted today with Holger from reproducible.debian.net and couldn't resist doing reproducible builds ;).
These patches need additonal work, but they are working when the git tree is clean.
You can test coreboot reproducible when building without payload. Or a more advanced way integrate my seabios git-tree
into coreboot's payload target.
coreboot needs 2 patches (one is very dirty ;) to build reproducible.
SeaBIOS requires one patch.