Search for Intel HAP (High Assurance Program). It's a magic killswitch that disables the ME, provided officially only to three-letter agencies and such, of course.

As for coreboot, keep in mind that it was originally developed long before the ME was a thing. IIRC the original motivation was that BIOS sucks. Which it does, sure, but that's orthogonal to the ME.

GP is referring to Intel's High Assurance Program (http://blog.ptsecurity.com/2017/08/disabling-intel-me.html) which offered the possibility to disable ME.