Improve support for Home Assistant OS forks

Maintaining Home Assistant OS on many platforms does not scale well: It slows down development of Home Assistant OS, makes it harder to move to newer Buildroot and Linux kernel versions, in short binds resources.

Today, the upper layer stack (Supervisor, Home Assistant Core etc.) are largely board agnostic. Their generic variants can run on any board and provide the regular Home Assistant experience without downsides. However, today the Home Assistant OS update system is integrated with Supervisor, which makes it hard to maintain a fork of Home Assistant OS while also provide updates for all the rest of the Home Assistant components.

To make it easier for the community to support other boards, and to use as a staging ground, this task aims to make it easier to create and maintain installations from Home Assistant OS forks.