sysext: provide systemd-{sysext,confext}-sysroot.service services

The new services are used to activate system and configuration extensions
for the main system from the initrd, this allows to overcome the limitation
that sysext/confext cannot be used to update the resources which are required
in the earliest boot of the system (before systemd-sysext/systemd-confext
start).

presets/90-systemd-initrd.preset

@@ -9,12 +9,14 @@
 
 # Settings for systemd units distributed with systemd itself, specific to initrds.
 
+enable systemd-confext-sysroot.service
 enable systemd-journald-audit.socket
 enable systemd-network-generator.service
 enable systemd-networkd.service
 enable systemd-networkd-wait-online.service
 enable systemd-pstore.service
 enable systemd-resolved.service
+enable systemd-sysext-sysroot.service
 enable systemd-tpm2-clear.service
 
 disable console-getty.service

presets/90-systemd.preset

@@ -52,7 +52,9 @@ disable proc-sys-fs-binfmt_misc.mount
 disable syslog.socket
 
 disable systemd-boot-check-no-failures.service
+disable systemd-confext-sysroot.service
 disable systemd-journal-gatewayd.*
 disable systemd-journal-remote.*
 disable systemd-journal-upload.*
+disable systemd-sysext-sysroot.service
 disable systemd-time-wait-sync.service