Unified cc2538 examples into single cc2538-common placeholder

examples/cc2538-common/mqtt-demo/README.md

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+MQTT Demo
+=========
+The MQTT client can be used to:
+
+* Publish sensor readings to an MQTT broker.
+* Subscribe to a topic and receive commands from an MQTT broker
+
+The demo will give some visual feedback with the green LED:
+* Very fast blinking: Searching for a network
+* Fast blinking: Connecting to broker
+* Slow, long blinking: Sending a publish message
+
+Publishing
+----------
+By default the example will attempt to publish readings to an MQTT broker
+running on the IPv6 address specified as `MQTT_DEMO_BROKER_IP_ADDR` in
+`project-conf.h`. This functionality was tested successfully with
+[mosquitto](http://mosquitto.org/).
+
+The publish messages include sensor readings but also some other information,
+such as device uptime in seconds and a message sequence number. The demo will
+publish to topic `iot-2/evt/status/fmt/json`. The device will connect using
+client-id `d:quickstart:cc2538:<device-id>`, where `<device-id>` gets
+constructed from the device's IEEE address.
+
+Subscribing
+-----------
+You can also subscribe to topics and receive commands, but this will only
+work if you use "Org ID" != 'quickstart'. To achieve this, you will need to
+change 'Org ID' (`DEFAULT_ORG_ID`). In this scenario, the device will subscribe
+to:
+
+`iot-2/cmd/+/fmt/json`
+
+You can then use this to toggle LEDs. To do this, you can for example
+use mosquitto client to publish to `iot-2/cmd/leds/fmt/json`. So, to change
+the state of an LED, you would do this:
+
+`mosquitto_pub -h <broker IP> -m "1" -t iot-2/cmd/leds/fmt/json`
+
+Where `broker IP` should be replaced with the IP address of your mosquitto
+broker (the one where you device has subscribed). Replace `-m "1'` with `-m "0"`
+to turn the LED back off.
+
+Bear in mind that, even though the topic suggests that messages are of json
+format, they are in fact not. This was done in order to avoid linking a json
+parser into the firmware. This comment only applies to parsing incoming
+messages, outgoing publish messages use proper json payload.
+
+IBM Quickstart Service
+----------------------
+It is also possible to publish to IBM's quickstart service. To do so, you need
+to undefine `MQTT_DEMO_BROKER_IP_ADDR`.
+
+The device will then try to connect to IBM's quickstart over NAT64, so you will
+need a NAT64 gateway in your network to make this work. A guide on how to
+setup NAT64 is out of scope here.
+
+If you want to use IBM's cloud service with a registered device, change
+'Org ID' (`DEFAULT_ORG_ID`) and provide the 'Auth Token' (`DEFAULT_AUTH_TOKEN`),
+which acts as a 'password', but bear in mind that it gets transported in clear
+text.