IT WORKS!!!
Well, not with Libreboot:
* keyboard fails to work after reboot unless removing CMOS
* it couldn't launch a good disk install.
* it couldn't reach the LiveCD after the first flashing of the rom on the
chip
With Coreboot (thanks to #coreboot):
* I built only for i386 since 64 bits isn't supported (it switches to 64 bits
when the OS boots up)
* "Graphics initialization" --> "None" in the Devices menu
(CONFIG_NO_GFX_INIT)
* in the Console menu deselect "Use onboard VGA as primary video device"
(CONFIG_ONBOARD_VGA_IS_PRIMARY)
I also updated my Coreboot sources, mine is nearly 10 months old...
I can reach the LiveCD, the install on disk (there is the fsck error
displayed, but I still can boot), and it seems stable.
So I'm gonna try it for the 6 cores after I've fixed VGA support (both
internal and external GPU). It doesn't work at all.