In #256226, `backdoor.service` was changed to be part of
`sysinit.target` instead of having default dependencies. This broke
several tests that relied on `backdoor.service` starting after default
targets. For example, `systemd-boot.update` expects `/boot` to be
mounted as soon as the backdoor is running.
These tests really ought to be declaring their dependencies properly
with things like `machine.wait_for_unit("local-fs.target")`, because
it's useful for the backdoor to start as early as possible. But for
now, let's just order it the way it was before in stage 2, and use the
earlier ordering in the new stage 1 context.
1a2f2cf95d