systemd: set SYSTEMD_SULOGIN_FORCE=1 for emergency.service

On error of the installer we invoke emergency.target. The executable
that gets called in emergency.service will won't give us a shell if
the root account is locked. We'll get a message like this:

```
Cannot open access to console, the root account is locked.
See sulogin(8) man page for more details.
```

We can force to login by setting SYSTEMD_SULOGIN_FORCE=1.

systemd/coreos-installer-generator

@@ -33,4 +33,9 @@ if [ -n "$(karg coreos.inst.install_dev)" ]; then
     if ! karg_bool coreos.inst.skip_reboot; then
         touch /run/coreos-installer-reboot
     fi
+
+    # Allow emergency.service to execute even with locked root account
+    mkdir -p /etc/systemd/system/emergency.service.d
+    echo -e '[Service]\nEnvironment=SYSTEMD_SULOGIN_FORCE=1\n' \
+        > /etc/systemd/system/emergency.service.d/override.conf
 fi