05core/autologin: query journal instead of /run/ignition.json

We shouldn't use `/run/ignition.json` to determine whether a user
config was provided since it's implementation details. Instead, use the
new official journal messages that Ignition emits.

Closes: #514

overlay.d/05core/usr/lib/systemd/system-generators/coreos-liveiso-autologin-generator

@@ -101,7 +101,9 @@ fi
 # If the user supplied an Ignition config, they have the ability to enable
 # autologin themselves.  Don't automatically render them insecure, since
 # they might be running in production and booting via e.g. IPMI.
-if [ -e /run/ignition.json ] ; then
+# See https://github.com/coreos/ignition/pull/958 for the MESSAGE_ID source.
+ign_usercfg_msg=$(journalctl MESSAGE_ID=57124006b5c94805b77ce473e92a8aeb IGNITION_CONFIG_TYPE=user)
+if [ -n "${ign_usercfg_msg}" ] ; then
     exit 0
 fi