DO NOT MERGE: iothub_ll_telemetry_sample to demo the SDK port

[LDong-Arm]

REPLACED BY #2

THIS PR IS TEMPORARY.
Before we have our own example, this PR demonstrates the Azure SDK with the existing iothub_ll_telemetry_sample.

To use,

• Fetch this PR and run mbed deploy --protocol ssh
• Go to https://portal.azure.com/ -> IoT Hub, click on the listed hub to open
• On the sidebar, Explorers -> IoT devices, click on the device
• Copy the value of "Primary Connection String" and set it as the value of "iothub_connection_string" in mbed_app.json
• If you use WiFi, set the SSID and password in mbed_app.json
• Compile & run with baud rate = 115200.

[LDong-Arm]

Superseded by our own example.