Document the worker_threads option.

Change-Id: Id7b18728284ff1e36450e09a18ba7c3cb67e5a22
Skip updating the recovery since this seems to be not implemented yet.

updating_recovery will always be False since source_recovery/target_recovery are not available.

Change-Id: Ie7b7807f6bb81af9706013968b3eb5bbf62d1bb2
Add a "device specific" option to skip recovery updates during incremental OTAs.

Change-Id: I15288b883c0cbd9fd1fbd84564a8c81dbcef73ec
Add a device-specific option to disable the installation of a boot-image.
Add a device-specific configuration for clearing the /system partition.

By default this will be formatted, but some locked bootloaders require
that it's only "recursively deleted".

Change-Id: Ia902b867f150ac0ff836d0a1280f7439217401ea