Implement working from a boot.img only

[samueldr]

There are multiple facets to this.

The main I have in mind is to get a recovery image (e.g. TWRP) and work from there.

The main drawback here is that props could be incomplete, compared to OEM props. Though this is not really an issue I suppose as we don't many props.

An additional benefit is this allows a user to extract a boot partition from a rootable device for which there are no system images. Not common, and not really useful since it's almost expected there won't be a kernel dump. Though at least it's possible.

It also would help people get started if they somehow have boot.img from a format that is not expanded yet.