Document the worker_threads option.

Change-Id: Id7b18728284ff1e36450e09a18ba7c3cb67e5a22

Conflicts:
	tools/releasetools/ota_from_target_files
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

Conflicts:
	tools/releasetools/ota_from_target_files
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

Conflicts:
	tools/releasetools/ota_from_target_files
Add missing "delete recursive" function.
Fixed boot image installation if FullOTA_DisableBootImageInstallation is enabled.

Change-Id: Id41e7117fb1ece55aa61ef74d63316c39277c6a7
Make it possible to override the whole "format /system" routine.

Deciding between "format" and "recursive delete" might not be enough for
some devices. Thus we let them override the whole formatting process.

Conflicts:
	tools/releasetools/ota_from_target_files
Bring back squisher as an optional part of the build

Some devices could benefit from the size reduction from running
squisher, so bring it back as an optional part of the build process.

Change-Id: I288f694fd1bf26782e05dcfa797a0569d9fdf9cb