[Bug 2080272] Re: install kernel in the minimal layer: server, desktop, flavors

[Dan Bungert]

** Merge proposal linked:
   https://code.launchpad.net/~dbungert/ubuntu-seeds/+git/ubuntu-seeds/+merge/472993

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Title:
  install kernel in the minimal layer: server, desktop, flavors

Status in livecd-rootfs package in Ubuntu:
  Triaged

Bug description:
  [Proposed Changes]

  I aspire to place the kernel in the minimal layer for Subiquity-based
  install ISOs.

  By placing the kernel in minimal, we can achieve the following
  improvements:

  1. Space savings - there are redundant packages present in the ship-live
     pool and in the live layer.  Adding the kernel to minimal means that
     the kernel is already in the live layer, and we don't then also need
     it in the pool.
  2. Time savings - informal vm testing suggests more than a minute
     improvement to have the kernel preinstalled over installing it at
     runtime.

  As always, there is a cost tradeoff:

  1. If a different kernel is desired, we need to be able to remove this
     preinstalled kernel.  Relevant curtin and subiquity changes are
     already landed.
  2. When installing that other kernel, it'll take longer than today due
     to still needing to install a kernel at runtime + the time cost of
     removing the preinstalled kernel.

  
  [Changes required]
  * livecd-rootfs changes to install kernel to the minimal layer instead
    of the live-layer, for all relevant install ISOs
  * seed changes to remove a kernel from the pool, for all relevant
    install ISOs

  
  [Test Strategy]
  I have employed ubuntu-old-fashioned [1] to run livecd-rootfs locally,
  taken the resulting outputs, and replaced those files on an oracular
  live-server ISO using livefs-editor [2].  I can then install using
  the resulting ISO and verify that the target system boots the intended
  kernel, be it the default one or an alternate of my choosing.

  [1] https://github.com/ubuntu-bartenders/ubuntu-old-fashioned
  [2] https://github.com/mwhudson/livefs-editor

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