Allow custom path for grub.cfg

[ttwd80]

Description

Add the ability to specify custom location for grub.cfg.

Why is this needed

Ubuntu 24.04 cloud image mounts the /boot partition. The default /boot/grub/grub.cfg location is invalid when we mount the partition. The location when there is a /boot partition is grub/grub.cfg.

Fixes: #142

How Has This Been Tested?

• Tested on code vagrant with libvirt from https://github.com/tinkerbell/playground/blob/main/stack/docs/quickstarts/VAGRANTLVIRT.md
• Test case 1 - Drop-in change with jammy. No GRUBCFG_PATH. I was able to ssh into [email protected].
• Test case 2 - Drop-in change with noble. No GRUBCFG_PATH. I was NOT able to ssh into [email protected]. Expected to fail as noble uses a /boot partition.
• Test case 3 - Drop-in change with noble. The value for GRUBCFG_PATH was set to grub/grub.cfg. The value for BLOCK_DEVICE was set to {{ formatPartition ( index .Hardware.Disks 0 ) 16 }}. I was able to ssh into [email protected].

How are existing users impacted? What migration steps/scripts do we need?

It uses the default value of boot/grub/grub.cfg for GRUBCFG_PATH if not specified. Existing users are not impacted.

Checklist:

I have:

• updated the documentation and/or roadmap (if required)
• added unit or e2e tests
• provided instructions on how to upgrade

More info on Ubuntu 24.04 cloud image partition - https://bugs.launchpad.net/cloud-images/+bug/2072929