Add GitHub action to build firmware

[mdkendall]

This PR adds a GitHub Action that automatically builds the firmware (in a Ubuntu virtual machine on GitHub's servers) whenever a commit is pushed to the GitHub repository. The output archive contains two files:

• tinygs-tagname-full.bin is a full image containing the second-stage bootloader, partition table and firmware itself. It is suitable for programming with the one-click uploader.
• tinygs-tagname-ota.bin contains just the firmware. It is suitable for over-the-air reprogramming.