Compared to conventional BIOS systems, Coreboot features different payloads for which it supports after initializing the key hardware components. Besides having the well known SeaBIOS, there's been some GRUB2 support, the FILO bootloader, Etherboot, and TianoCore, among other instances. However, with the latest Coreboot Git activity, GRUB2 is now officially supported with its build system.
With this Git commit, GRUB2 has been added an official payload for the build system:
Since a long time GRUB 2 is a viable payload alternative to SeaBIOS and FILO. So make it easy for coreboot users to use GRUB 2 as a payload by integrating it into coreboot’s build system, so it can be selected in Kconfig.
As the last GRUB 2 release 2.00 is too old and has several bugs when used as a coreboot payload only allow to build GRUB 2 master until a new GRUB release is done. The downside is, that accidental breakage in GRUB’s upstream does not affect coreboot users.
Currently the GRUB 2 payload is built with the default modules which results in an uncompressed size of around 730 kB. Compressed it has a size of 340 kB, so it should be useable with 512 kB flash ROMs.