Support Node.js app embeddding as part of configuration web UI

[aniongithub]

Since Balena Etcher is a Node.js app, embed it into our configuration web UI. This will allow users to use a generic pi-bootstrap image to write other Raspberry Pi images.

On second thought, maybe the configuration UI could be a separate repo cloned as a submodule so users can customize it as needed. Nevertheless we need to ensure our Captive Portal workflow and configuration web UI are Node.js app compatible, not just PHP or static HTML.