This is impossible without cooperation from intel, any intel system from
nehalem (first core system) on will NOT work without the ME binary
blobs, the (black box) supervisor processor will simply auto-restart
after 30 minutes.
If google can't get intel to release the code and a flashing mechanism
for ME then nobody can.
https://web.archive.org/web/20161010040458/https://blogs.coreboot.org/blog/2015/02/23/the-truth-about-purism-why-librem-is-not-the-same-as-libre/
ME, PSP, FSP, etc containing firmware is not "open source" as binary
blobs perform the entire boot process with coreboot simply a wrapper layer.
For brand new computers the only real choices are POWER and (some) ARM.
All current IBM POWER systems are entirely owner controlled with no
hardware signing key enforcement and you can buy one starting at around
3K (comparable to an intel server system of equivalent cost).
if you want an x86-64 laptop the only choice is a 2013 FM2 processor (no
AMD PSP unlike FM2+) which is new enough to be useful (see coreboot on
the lenovo G505S)
For x86-64 desktops there is the KGPE-D16 and KCMA-D8 coreboot
motherboards which will work without blobs, which are both new enough to
be useful but not brand new.