There was a regression introduced by the change to use
systemd-systemctl-native rather than a python fake implementation,
which caused template units to not be properly enabled when set in
the SYSTEMD_SERVICE variable. Through investigation, it seems that
the best way to re-enable template instances is to handle them
explicitly in the systemd.bbclass and enable them with `preset`, like
most units are handled[1,2].

Per the systemd.preset manpage, the format for template units is
different than for regular units[3]. We need to coalesce all the
template instances onto a single line and emit them as an additional
space-deliminated argument.

Ran this against openbmc's phosphor-ipmi-net recipe and generated
the following preset file:
```
$ cat packages-split/phosphor-ipmi-net/usr/lib/systemd/system-preset/98-phosphor-ipmi-net.preset
enable phosphor-ipmi-net@.service eth0
enable phosphor-ipmi-net@.socket eth0
```

Thanks again Patrick for looking into this. I can confirm that the patch resolves the issue I encountered - hoping for it to be backported to Walnascar after merge.