Draft: Add systemd to pmOS edge

This adds the systemd repo to pmOS edge, allowing folks to (among other things) build pmOS images with systemd more easily.

Currently we are requiring this to be merged before inviting more folks to test systemd on pmOS (see: #2938)

Fixes: #2773

Replaces: !4956 (closed)

How to review this

NOTE: It's probably not worth reviewing this until all of the TODOs below are done.

There are a lot of commits in this branch/MR, I don't expect everyone to review them all... they have been reviewed when they were merged into the staging branch.

If you want some suggestions about what would be useful to review for this MR, ask me and I'll add it here.

TODO

• Go through !4956 (closed) and make sure that anything still relevant there is added to the staging branch
• Remove "not for master" commits
  • CI: build_changed_aports: systemd staging (MR 5070)
    • Half of this patch can be dropped when this is merged: !5576 (merged)
    • NOTE: drop part of patch that sets the mirror right before merging this branch
  • CI: hardcode get_upstream_branch (MR 5070)
    • NOTE: drop entire patch right before merging this branch
  • CI: disable armhf, riscv64, x86 (MR 5070)
    • NOTE: drop entire patch right before merging this branch
  • remove temp/* pkgs that conflict with systemd repo
    • Depends on build.postmarketos.org#140
    • Also see: build.postmarketos.org#143 (closed)
  • temp/mutter-mobile: drop
  • temp/gnome-settings-daemon-mobile: drop
  • CHECK: systemd/xorg-server: new aport
  • CHECK: systemd/networkmanager: new aport
• Resolve linked dependency issues/MRs
• pmbootstrap patch: pmbootstrap!2405 (merged) (needed for bpo to split the repo)
• bpo changes, so this does not get built as part of the regular repository when merged (build.postmarketos.org#140)
• fix apk-tools /usr merge breaking upgrades: #3202 (closed)
• systemd: revert patches for splitting /usr (#3234)