CI/CD pipeline for on-device unit tests

[zhovner]

We need a way to automatically test changes to our key codebases using the dev boards in our test farm (those attached to the London build rack and any others we may want to link in)

• Auto-deploy each of the freshly built images from Buildbot to supported boards
• Add a minimal PoC unit test for all boards (e.g. “boots to Debian serial prompt”)
• Add a way to test pre-moderated pull requests from our Github repos
• Document how additional unit tests should be added
• Document how the whole thing works