use full-path simlinks in unit activation

The following discussion from !79 (merged) should be addressed:

• @nl6720 started a discussion:

  Currently other custom units (choose-mirror.service and pacman-init.service) use relative links. Let's be consistent.

i think that full path links is more convenient. since they allow you to use

systemctl enable UNIT --root=

option. @dvzrv what do you think?

added hintgood-first-issue scopeenhancement labels

set weight to 2

Hm, it's not a bad idea tbh, but it also requires us to a) have a systemd based system as build host (not super problematic, as we only support Arch directly at this point in time, but would be nice to have as few constraints as possible) and b) requires us to actually flesh out the way we want to do declarative profiles much more first (see e.g. #29 (closed), #38 (closed), #61 (closed)).

So not really a can of worms we can open right now. But we can keep this open for the future.

systemctl --root= doesn't require systemd to be running.

https://jlk.fjfi.cvut.cz/arch/manpages/man/core/systemd/systemctl.1.en

--root=

When used with enable/disable/is-enabled (and related commands), use the specified root path when looking for unit files. If this option is present, systemctl will operate on the file system directly, instead of communicating with the systemd daemon to carry out changes.

But all that's irrelevant anyway. You can only use systemctl --root= to enable custom systemd units that are placed in the airootfs directory. It will not work for units that are packaged since the unit files will not be there; to enable those you still need to construct the symlinks manually. So IMHO this issue basically comes down to aesthetics.