The user can select the cloud connection to connect the application to.
Either AWS IoT or Azure IoT Hub.

To add Azure support, an Azure device must be created following.
Then the device connection string must be added into credentials.
Finaly, to build with Azure support compile with the flag -e AZURE.

On the technical side, the Azure code and AWS code have been split
with their associated app code into two different files:
- aws_demo.c
- azure_demo.c

The AWS code remains as it is before this addition while the azure code
follows closely the example exposed by the Open IoT SDK.

The CI pipeline has been modified to support and test Azure. While
compiling more examples, it reuse build trees and is therefore faster
than the previous pipeline.

Signed-off-by: Vincent Coubard <vincent.coubard@arm.com>