There is also the possibility that AMD may in the future support Coreboot. China's "licensed" copy of EPYC is working on it, and as those copies are, well, direct copies of EPYC, there's no reason it can't work on AMD's designs.
https://www.phoronix.com/scan.php?page=news_item&px=Hygon-Dhyana-Coreboot-First