changed the platform from photon to duo

[paragshar]

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Doneness:

• [x] Contributor has signed CLA
• [x] Problem and Solution clearly stated
• [x] Code peer reviewed
• [x] API tests compiled
• [x] Run unit/integration/application tests on device
• [x] Add documentation
• [x] Add to CHANGELOG.md after merging (add links to docs and issues)

[XuGuohui]

Closed because this repo will be merge to upstream: https://github.com/spark/firmware

Thanks anyway!