Feature/basic free rtos

[elsalahy]

Summary:

Add FreeRTOS support to our node Relates to #42

Changes:

• Adds FreeRTOS kernal as a submodule
• Adds a basic low power app with multiple threads

Notes for Reviewers:

Tested, but I'm still improving the structure of the files.

Release Notes: (optional)

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[elsalahy]

Can someone give this a quick review, should be simple.

[elsalahy]

@azerimaker good catch, I fixed it, can you check again and let me know if you notice something else

@johanstokking do you have some feedback on this?