systemd: use bootconfig for unified cgroup hierarchy

[bcressey]

Issue number:
Related: #3883

Description of changes:
This takes advantage of the new Twoliter support for bootconfig drop-ins and variant metadata.

Variants with the unified-cgroup-hierarchy feature flag set will end up installing the new subpackage when the image is built, which will populate the default bootconfig with the right systemd option.

Default to "hybrid" at build time rather than "unified" since the two variants that don't support boot config (aws-ecs-1 and aws-ecs-1-nvidia) also require "hybrid" mode.

Testing done:
When building aws-dev, the following capability is logged for the metadata rpm:

  #13 0.582 bottlerocket-image-feature(unified-cgroup-hierarchy)

Later, this triggers the expected installation of the new subpackage:

  #15 1.175 Installing dependencies:
...
  #15 1.175  bottlerocket-systemd                          aarch64 252.22-1      repo 4.4 M
  #15 1.175  bottlerocket-systemd-console                  aarch64 252.22-1      repo  49 k
  #15 1.175  bottlerocket-systemd-networkd                 aarch64 252.22-1      repo 1.6 M
  #15 1.175  bottlerocket-systemd-resolved                 aarch64 252.22-1      repo 416 k
  #15 1.175  bottlerocket-systemd-unified-cgroup-hierarchy aarch64 252.22-1      repo 7.2 k

After boot, the expected hierarchy and settings are present:

bash-5.2# df /sys/fs/cgroup
Filesystem     1K-blocks  Used Available Use% Mounted on
cgroup2                0     0         0    - /sys/fs/cgroup

bash-5.2# apiclient get settings.boot.init
{
  "settings": {
    "boot": {
      "init": {
        "systemd.unified_cgroup_hierarchy": [
          "1"
        ]
      }
    }
  }
}

I also verified I could override the setting and the system would come up in hybrid mode.

For a variant like aws-k8s-1.24, the hybrid subpackage is installed and the system comes up in hybrid mode as expected.

  #15 1.191 Installing dependencies:
...
  #15 1.191  bottlerocket-systemd                         aarch64 252.22-1       repo 4.4 M
  #15 1.191  bottlerocket-systemd-hybrid-cgroup-hierarchy aarch64 252.22-1       repo 6.6 k

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[bcressey]

⬆️ force push removes the use of Supplements and weak dependencies. It's not quite as elegant but will avoid the risk of subtle failures.

[bcressey]

⬆️ force push fixes parens

[yeazelm]

Nice!