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: 22e9053086da57d1a300a179bba32c4cf09e0c10)

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes-recipe/systemd.bbclass

@@ -224,6 +224,8 @@ python systemd_populate_packages() {
                         service, pkg_systemd, "Also looked for service unit '{0}'.".format(base) if base is not None else ""))
 
     def systemd_create_presets(pkg, action, user):
+        import re
+
         # Check there is at least one service of given type (system/user), don't
         # create empty files.
         needs_preset = False
@@ -239,10 +241,17 @@ python systemd_populate_packages() {
         presetf = oe.path.join(d.getVar("PKGD"), d.getVar("systemd_unitdir"), "%s-preset/98-%s.preset" % (prefix, pkg))
         bb.utils.mkdirhier(os.path.dirname(presetf))
         with open(presetf, 'a') as fd:
+            template_services = {}
             for service in d.getVar('SYSTEMD_SERVICE:%s' % pkg).split():
                 if not systemd_service_exists(service, user, d):
                     continue
-                fd.write("%s %s\n" % (action,service))
+                if '@' in service:
+                    (subservice, instance, service_type) = re.split('[@.]', service)
+                    template_services.setdefault(subservice + '@.' + service_type, []).append(instance)
+                else:
+                    fd.write("%s %s\n" % (action,service))
+            for template, instances in template_services.items():
+                fd.write("%s %s %s\n" % (action, template, ' '.join(instances)))
         d.appendVar("FILES:%s" % pkg, ' ' + oe.path.join(d.getVar("systemd_unitdir"), "%s-preset/98-%s.preset" % (prefix, pkg)))
 
     # Run all modifications once when creating package