systemd.bbclass: generate preset for templates

systemd.bbclass: generate preset for templates

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
```

[1]: https://lore.kernel.org/openembedded-core/Z2ch.1747051947055246176.oktf@lists.openembedded.org/
[2]: https://lore.kernel.org/openembedded-core/aDdoTVtCmElpURYD@heinlein/
[3]: https://www.freedesktop.org/software/systemd/man/latest/systemd.preset.html

Fixes: 7a580800db39 ("systemd: Build the systemctl executable")
(From OE-Core rev: f33d9b1f434e40a459614d8dc21ce45e11581008)

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>