Add script to create and install avahi service unit

Intended to setup Zeroconf/mDNS for HA discovery

script/gen-esphome-avahi.sh

@@ -0,0 +1,33 @@
+#!/usr/bin/env bash
+set -euo pipefail
+PORT=6053
+NAME="%h (Linux Voice Assistant)"
+
+mapfile -t IFACES < <(ip -o link show up | awk -F': ' '{print $2}' | grep -v '^lo$')
+
+TXT="    <txt-record>platform=linux</txt-record>
+"
+LIST=""
+for i in "${IFACES[@]}"; do
+  mac=$(cat "/sys/class/net/$i/address")
+  if [ -d "/sys/class/net/$i/wireless" ] || [[ $i == wl* ]]; then net=wifi; else net=ethernet; fi
+  TXT+="    <txt-record>mac_${i}=${mac}</txt-record>
+"
+  TXT+="    <txt-record>net_${i}=${net}</txt-record>
+"
+  LIST+="${i},"
+done
+LIST=${LIST%,}
+
+cat >/etc/avahi/services/esphome-lva.service <<XML
+<?xml version="1.0" standalone='no'?><!--*-nxml-*-->
+<!DOCTYPE service-group SYSTEM "avahi-service.dtd">
+<service-group>
+  <name replace-wildcards="yes">${NAME}</name>
+  <service>
+    <type>_esphomelib._tcp</type>
+    <port>${PORT}</port>
+    <txt-record>ifaces=${LIST}</txt-record>
+${TXT}  </service>
+</service-group>
+XML