[Bug 2109822] Re: Add support for building ARM64 images with nvidia kernels in noble

[Antoine Lassagne]

** Merge proposal unlinked:
   https://code.launchpad.net/~cypress-team/livecd-rootfs/+git/livecd-rootfs/+merge/486093

** Merge proposal unlinked:
   https://code.launchpad.net/~cypress-team/livecd-rootfs/+git/livecd-rootfs/+merge/487545

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https://bugs.launchpad.net/bugs/2109822

Title:
  Add support for building ARM64 images with nvidia kernels in noble

Status in livecd-rootfs package in Ubuntu:
  Fix Released
Status in livecd-rootfs source package in Noble:
  In Progress
Status in livecd-rootfs source package in Plucky:
  In Progress
Status in livecd-rootfs source package in Questing:
  Fix Released

Bug description:
  [ Impact ]

   * This is a hardware enablement SRU.

   * In partner engineering, we maintain specific nvidia hardware which require a dedicated optimized kernel. We need to build Desktop images for these machines, on a regular basis, to run the certification suites.
  Right now the kernel we need is `linux-nvidia-hwe-24.04-edge`, but I'm anticipating that it will be `linux-nvidia-hwe-24.04` at some point so this should be possible to configure.

   * The resolution involves adding a subarchitecture (SUBARCH) option
  in Desktop images which, when provided, will behave accordingly. a
  CHANNEL option can also be provided to add the channel.

  [ Test Plan ]

   1. Create a Desktop ISO. It will contain the generic kernel.
   2. Create a Desktop ISO with the proposed package. It will contain the generic kernel too.
   3. Create a Desktop ISO after mentioning subarch=nvidia in the metadatas. The kernel should now be nvidia's.
   4. Create a Desktop ISO after mentioning subarch=nvidia in the metadatas. The kernel should now be nvidia's edge.
   5. Build the images from 3, 4, 5 with ubuntu-cdimage and test that they boot correctly and have the right kernel.
  Please not that the debian-cd branch to be used it the following merge proposal: https://code.launchpad.net/~antoinelassagne/debian-cd/+git/debian-cd/+merge/485191
   6. Redo all these steps with Live Server image

  [ Where problems could occur ]

   * If the bash scripts becomes invalid or go in the wrong branch, the
  image build process could simply stop working, or provide images with
  the wrong kernel. The test plan reflects this risk.

  [ Other Info ]

   * The kernel that are made available with this fixed are only released on LTS series right now. To test it on non LTS series, we'd have to package the kernel into a private PPA before.
   * Nevertheless, a MP for plucky was added to fulfill the SRU process in regard to apply patches from newest to oldest release.
   * But skipping 'Oracular' since it reaches it's end of live very soon (and to avoid extra effort, since plucky is a newer non-LTS).
   * But most relevant is to have the patch in the latest LTS / noble.

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