core user: specifically enable [email protected]

This enables `[email protected]`, which is the default
instance of `[email protected]` when the `core` user is
created in `base.ign`.

overlay.d/05core/usr/lib/dracut/modules.d/40ignition-conf/README.md

@@ -0,0 +1,7 @@
+FCOS enables `[email protected]` from `base.ign`, allowing the user
+to prevent Ignition from enabling the service with a user config if the user
+wants to change the username. Unlike FCOS, RHCOS doesn't fetch SSH keys and thus
+doesn't need `[email protected]`. Therefore, RHCOS maintains its
+own copy of `base.ign`, and changes to one copy need to be synced to the other
+copy.
+See https://github.com/coreos/fedora-coreos-config/pull/626

overlay.d/05core/usr/lib/dracut/modules.d/40ignition-conf/base.ign

@@ -15,5 +15,13 @@
         ]
       }
     ]
+  },
+  "systemd": {
+    "units": [
+      {
+        "enabled": true,
+        "name": "[email protected]"
+      }
+    ]
   }
 }

overlay.d/05core/usr/lib/systemd/system-preset/40-coreos.preset

@@ -16,8 +16,6 @@ enable afterburn-checkin.service
 enable afterburn-firstboot-checkin.service
 # Target to write SSH key snippets from cloud providers.
 enable afterburn-sshkeys.target
-# Service to write SSH key snippets from cloud providers.
-enable [email protected]
 # Update agent
 enable zincati.service
 # Testing aid