OK, removing some RAM made the boot faster, at least. But still the same issues otherwise.
So I tried Libreboot again, and while the boot time is slow, it gives the best results: it goes beyond the SeaBIOS screen on the disk install (and the LiveCD works fine). But I land on a blank screen with a blinking underscore. I'm investigating that right now.
The install on disk was still failing, but eventually it ended OK (with GRUB installed, though I don't need it, and without network updates because it kept on failing earlier).
So, provided I find a way to boot all the way, and that Libreboot seems to work, then I must be doing something wrong with Coreboot, though I've read a lot of doc, a lot of help in "make menuconfig".
If someone has a clue, I'm interested. The Coreboot boot time is way better than Libreboot's.
