Rework initrd usr mount to start Ignition/Afterburn from there

[pothos]

• Rework initrd usr mount to start Ignition/Afterburn from there

  The size of the unified kernel image grew so much that we almost hit
  the limit of the fixed /boot partition. While we could tweak which
  kernel modules are present the largest contributors are actually
  Ignition and Afterburn (coreos-metadata). Since we also ship Afterburn
  on the /usr partition and have Ignition installed there, too (but
  currently masked), we could do what was done for Torcx and directly
  call it from there (but with LD_LIBRARY_PATH instead of chroot). There
  are a few corner cases where this was not directly possible due to the
  order of events in the initrd and these had to be reworked.

  This establishes a structure that allows the /sysusr/usr mount to be
  used for calling Ignition and Afterburn. This mount point was set up
  by systemd already but since we have a legacy generator, this needed
  adaption, too. The final /usr mount in /sysroot/usr is still set up.
  In the case of the systemd generator it is a bind mount, for the
  legacy generator it is a separate mount, both work but we could align
  this to also use a bind mount. The afterburn-network-kargs.service
  starts a bit later now because it depends on the /sysusr mount and
  this also means that the parse-ip-for-networkd dracut hook needed to
  run at a later stage, here solved with an own service but since the
  dracut hook mechanism with sourcing is complicated, the script still
  runs as hook to serialize the environment. The fsck check for /usr was
  also in the way and thus we write out a dummy systemd-fsck-usr.service
  since we don't need to check /usr as it's verity-protected. In the end
  with these changes we significantly reduce the unified kernel image
  size and have more breathing room for the next years. If we have to
  move some other binaries or even kernel modules that are not essential
  for loading /usr itself, we can move them out of the initrd based on
  this work. Size reduction for arm64 is 58 MB -> 52 MB, for amd64 it is
  55 MB -> 49MB.

• Align check for initrd-provided /usr

• Fix non-working norecovery option
  The check for the norecovery option used the same condition but had
  different actions in the if branch. Also, for the systemd generator
  the unit name was wrong because it was just the unit name of the bind
  mount.
  Align the check for the norecovery option and also use the right unit
  name.

How to use

Testing done

CI run

Tested the usr= cmdline case manually, tested the first boot ip=10.0.2.15::10.0.2.2:255.255.255.0::eth0:off:: cmdline case (and once with netroot=dummy) and ran the CI to test the kargs VMware case and that Ignition works.