[Bug 1961760] Re: `disk-image` not granular enough, often builds unused artifacts

[Ivan Kapelyukhin]

TEST CASE:
1. modified an image's series file to use disk-image-uefi instead of disk-image
2. built an image

VERIFICATION DONE
Image is built successfully, BIOS disk image isn't built.

** Tags removed: verification-needed verification-needed-impish
** Tags added: verification-done verification-done-impish

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https://bugs.launchpad.net/bugs/1961760

Title:
  `disk-image` not granular enough, often builds unused artifacts

Status in livecd-rootfs package in Ubuntu:
  Fix Released
Status in livecd-rootfs source package in Bionic:
  New
Status in livecd-rootfs source package in Focal:
  New
Status in livecd-rootfs source package in Impish:
  Fix Committed

Bug description:
  [Impact]

   * Due to how `disk-image` series file in `ubuntu-cpc` is structured,
  it builds BIOS and UEFI images at the same time. However, certain
  images (e.g., GCE images) require only UEFI image to be built, BIOS
  image is being simply discarded. This results in longer build times.

  [Test Plan]

   * Build any image that depends on `disk-image` (e.g. a GCE image) with `livefs-build` or `ubuntu-bartender`
   * Read build logs and observe what build artifacts are produced: `disk.ext4` is the BIOS image and `disk-uefi.ext4` is the UEFI image

  [Where problems could occur]

   * Due to changes in the `disk-images` series file the order in which
  BIOS and UEFI image are built has changed, if there's something
  sensitive to the order in which those artifacts are built it might
  break

  [Other Info]
   
   * CPC images will be switched from `disk-image` to `disk-image-uefi` once the update is released

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