[Bug 1932542] Re: Add support for zstd

[Łukasz Zemczak]

Thanks for the verification! Even though I REALLY hate releasing things
early, with the nature of this package and the SRUed changes, I think we
can do it conditionally - just this one time - to unblock image builds
for impish. Please be sure to keep your eyes open for any weird bug
reports on focal.

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https://bugs.launchpad.net/bugs/1932542

Title:
  Add support for zstd

Status in initramfs-tools package in Ubuntu:
  Fix Released
Status in initramfs-tools source package in Focal:
  Fix Released

Bug description:
  [Impact]

   * initramfs-tools in impish has changed default initrd compression to
  zstd. To allow compressing and uncompressing such initrds on Focal LTS
  release we should backport zstd support in the mkinitramfs &
  unmkinitramffs tooling. For example ubuntu-cdimage uses unmkinitramfs
  tool to unpack and inspect impish initrds on a focal host.

   * Cherrypicking the feature from 0.138 release, which is also present
  in Hirsute and Impish.

  [Test Plan]

   * Create initrd with zstd $ mkinitramfs -c zstd -o zstd-initrd.img

   * Uncompress it with $ unmkinitramfs ./zstd-initrd.img zstd-initrd-
  unpacked

  [Where problems could occur]

   * This adds support for zstd compressed initrd in both mkinitramfs
  and unmkinitramfs. However our GA kernels do not support zstd
  compressed initrds. And depends on zstd package is not added either.
  Meaning one still has to install zstd to gain the unpack support.
  Changing initramfs.conf to zstd may result in unbootable instance when
  used with GA kernel.

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