Customizations for supporting the Keybow

[doughsay]

Note: This PR is not to be merged. It's just here as a reminder of the branch that we build from and an easy place to view the diff.

To use this system in a nerves project

• Add this to your deps:

{:nerves_system_keybow,
 github: "nerves-keyboard/nerves_system_keybow",
 branch: "keybow",
 runtime: false,
 targets: :keybow}

To update to latest upstream

• Rebase this branch onto latest upstream/main

To add new features

• Push new commits to this branch

To create a new release

• mix nerves.artifact
• Create a new GitHub release pointing at this branch
• Tag it with vX.Y.Z+keybow.U (whatever is in the VERSION file in this system, e.g.: 1.20.0+keybow.1)
• Upload the generated file to the GitHub release

[doughsay]

See #2