Rather than inserting the logic to handle BIOS vs UEFI partition numbering for Clonezilla into an update-grub script, populate it at install time via the preseed/late_command.

This has the added bonus of seamlessly working for anyone who sets up a dual-boot or other non-default partition layout - as long as they still have a swap partition, a / partition, and a /boot/recovery partition.
Merge branch 'master' of https://github.com/directhex/steamos-installer