confext: add initrd-specific unit

In the initrd we want to run as early as possible, before
any of the filesystems are set up, so that users can use
confexts to customize fstab/veritytab/crypttab/etc. But
in the root fs it needs to run after /var/ has been set
up. Split the unit, and have an initrd-specific one that
runs very early.

units/meson.build

@@ -287,6 +287,11 @@ units = [
           'file' : 'systemd-confext.service',
           'conditions' : ['ENABLE_SYSEXT'],
         },
+        {
+          'file' : 'systemd-confext-initrd.service',
+          'conditions' : ['ENABLE_INITRD', 'ENABLE_SYSEXT'],
+          'symlinks' : ['initrd.target.wants/'],
+        },
         {
           'file' : 'systemd-coredump.socket',
           'conditions' : ['ENABLE_COREDUMP'],