Feature/AzureRtos

[elsalahy]

Summary:

This PR adds support for the awesome AzureRTOS (threadx) Closes #165

This paves the way for the LoRaWAN example in #166

Changes:

• Adds threadx submodule
• Adds a basic_azurertos application

Notes for Reviewers:

...

Release Notes: (optional)

Add AzureRTOS support

[elsalahy]

@johanstokking feel free to give it a quick look, but this is mainly just a notification for you to be aware of the new support.

[elsalahy]

@marnixcro I reduced the number of commits and did some minor docs improvements, please give it an approve when you can.