Grub2 preview

Depends on https://github.com/coreos/vboot_reference/pull/19

This code does not make any significant change our actual images, much more testing and a new version of grub is required first. This does get us most of the way there. For now I'm configuring grub to re-use the syslinux configuration so we can try the new bootloader before making changes to all the bits of code that poke at bootloader configuration.