finalize-staged: Ensure /boot automount doesn't expire

finalize-staged: Ensure /boot automount doesn't expire

If `/boot` is an automount, then the unit will be stopped as soon as the
automount expires. That's would defeat the purpose of using systemd to
delay finalizing the deployment until shutdown. This is not uncommon as
`systemd-gpt-auto-generator` will create an automount unit for `/boot`
when it's the EFI System Partition and there's no fstab entry.

To ensure that systemd doesn't stop the service early when the `/boot`
automount expires, introduce a new unit that holds `/boot` open until
it's sent `SIGTERM`. This uses a new `--hold` option for
`finalize-staged` that loads but doesn't lock the sysroot. A separate
unit is used since we want the process to remain active throughout the
finalization run in `ExecStop`. That wouldn't work if it was specified
in `ExecStart` in the same unit since it would be killed before the
`ExecStop` action was run.

Fixes: #2543