add serial_prefix to config

devices/serial_no.yaml

@@ -0,0 +1,9 @@
+text_sensor:
+  - platform: template
+    name: 'Serial No.'
+    lambda: |-
+      std::string mac = get_mac_address();
+      return to_string("${serial_prefix}") + mac.erase(0, mac.length()/2);
+    icon: mdi:expansion-card-variant
+    entity_category: diagnostic
+    update_interval: 60min