im on their mailing list for a long while now, and i know they have a sad battle… (even if i delete most of their mails, and i might have errors in the info i can tell u, i hope others will correct me wherever needed… 
also, maybe i know purism from there 
)
so the original firmware is locked down more and more all the time (check out me_cleaner and libreboot for more enlightemnent! ), coreboot isnt even the real core, but makes use of some central component to build their stuff around it, the real magic takes place in that component, im not sure if it is just not replacable or a blackbox binary or what else, maybe it is edk2 (maybe thats something completely different, it is hard to catch up even with their terminology, but it is on github) and i can also imagine that part as a highly customized android kernel, that u just cant recompile from upstream without whole subsystems becoming unusable… its like the baseband, that is the main software (firmware) that makes a phone a phone… they have binary blobs all around for a lot of things, like the blackbox drivers in android kernels, and at least the interface of those blobs should be known in order to be used as-is… there are some major vendors around it who collab there on their part and some dont even care, some try very hard to liberate things… some people are hired with nda’s who can access more juicy knowledge, but cant tell it to the rest of the world… intel, amd and maybe others dont really try hard to liberate all their swag, they want money and they want to hold their ranks… their users should be happy with what they get…
so now the best hope most open-everything warriors have is in risc-v, a young architecture (~10yrs old, and the implementations took years to become reality and useful and available and supported…), which is already everywhere, loosely controlled (any1 can extend(/alter) it, nobody knows all the vendors who produce it, it is in nvidia cards, esp boards, wd storages…), u can get open hw/fw/sw stuffs stuffed with it (haha ), but the concept of open isa is something else than what most ppl think about it, it is a vendor goodness instead of a user goodness… its power is in leaving behind the legal games of extending arm processors, that takes like 3 months for each new extension for the chips, which is too big time for this industry… if you want more horsepower and support, then the old players with their downsider are for you, if you want liberal computing, and trust in your hw (according to the docs of these; and in chinese vendors), and if u r willing to be more involved in solving issues, then risc-v is the way…
there was a dev at purism, his (i hope i say/assume it right, but im too oldschool) nick was the other name of Son Goku, i dont remember, i loved to read his blog posts and forum stuffs around purism, his job was to liberate the intel firmware, and he is a talented reverse engineer, but then there was a point with a component that is under hufman encoding, where they (not only him) could reach more and more %, like somewhere around 60-80, which means that this is the amount of that component that could be known at least, which is still not about what and why it does, but plain how, without help… so our dear rockstar freedom figter had to give it up or something like…
now u have an idea about coreboot…