fix: exclude Mirage kernel templates from updates

When targeting all templates to be updated, it fails on unsupported
templates. Avoid having a non-zero exit code.

salt/sys-mirage-firewall/create.sls

@@ -112,6 +112,9 @@ prefs:
 - kernel: mirage-firewall
 - kernelopts: ""
 - include_in_backups: False
+features:
+- enable:
+  - skip-update
 {%- endload %}
 {{ load(defaults) }}