Updating boot support partitions as required
In the discussion that follows, “sector” refers to a 512-byte disk sector, addressed by its Logical Block Address (LBA).Although the UEFI specs allow for disk sectors of other sizes, in practice 512 bytes is the norm.Note that different EFI BIOSes may have different requirements for the pathname of the bootloader.Most EFI BIOSes contain a "Compatibility Support Module" component which makes them act like legacy BIOSes, so they may boot either way.EFI BIOSes will use /boot/vmlinuz from the target rootfs.
Bootable Chromium OS drives (removable or not) share a common drive format.
On x86 we use Coreboot to set up RAM and load U-Boot.
You can find an overview of the verified boot process in the U-Boot Porting Guide.
The Chromium OS build process places GPT-aware boot sector code from syslinux in the MBR.
That code can specify one GPT partition to boot, indentified by a matching Unique Partition GUID field in the Partition Entry Array. The second-stage syslinux bootloader is installed on that partition, along with its corresponding config file (/syslinux/syslinux.cfg).