Provide options for manifest generation

[gprossliner]

While building the board filesystem, a /usr/manifest.json is unconditionally deployed to the board. This is a good feature to have, because my individual workflow needs that file for Over the Air Update.

{
  "version": "develop",
  "files": [
    {
      "file_name": "/usr/main.py",
      "size": 66,
      "integrity": "sha256-xVBVQEepGe4cZhEcqMZPjhCC3Gr/hoLvsplkNSRiIZI="
    }
}

Other projects may use different techniques or formats, so we should at least:

• Make generation of that file optional
• Change the name of the target file
• OptIn specific features like SNI Hash Generation