Actually, the latest version of Libreboot, which comes preinstalled on these
laptops, uses a custom payload called "SeaGRUB". It's SeaBIOS, configured to
immediately load GRUB after 0s, without allowing any other option, where both
SeaBIOS and GRUB are in the flash chip alongside coreboot-libre (the coreboot
version that libreboot uses).
In the GRUB menu of Libreboot, you can use BIOS services such as chainload
and so on. This is commonly used for booting non-GNU+Linux systems, such as
various BSD systems.
To the normal user, it just looks like a normal GRUB payload but it is in
fact SeaGRUB as described above.
This was done in the current Libreboot release, as a compromise so that BIOS
services were available to the user. In the Git repository, we have a
substantially improved version of this which uses BIOS GRUB compiled to a
floppy image (.img file) stored in the flash chip, where SeaBIOS still loads
GRUB immediately, but SeaBIOS initializes the video display instead of GRUB,
using the libre SeaVGABIOS module in SeaBIOS... basically this further
improves compatibility with other free OSes besides GNU+Linux. Andrew Robbins
and Swiftgeek on #libreboot IRC worked on it, and it will be in the next
release of Libreboot.
