Add mqtt example

mqtt/boot.py

@@ -0,0 +1,23 @@
+# This file is executed on every boot (including wake-boot from deepsleep)
+import esp
+esp.osdebug(None)
+
+import network
+import json
+
+with open("config.json", 'r') as f:
+    config = json.load(f)
+    
+print(config)
+
+sta_if = network.WLAN(network.STA_IF)
+if not sta_if.isconnected():
+    print('connecting to network...')
+    sta_if.active(True)
+    sta_if.connect(config["wifi"]["ssid"], config["wifi"]["password"])
+   
+while not sta_if.isconnected():
+    pass
+
+print('network config:', sta_if.ifconfig())
+