build: Add the factorypackage target

The build system generates several artifacts, among them update zips,
OTAs and update packages for fastboot.

Shipping devices typically need extra-special packages in order to
fulfill factory automation requirements.

This patches adds a "factorypackage" target that can be customized
by devices by providing a script that converts target files into the
desired format. The script path should be set into
TARGET_RELEASETOOL_FACTORY_FROM_TARGET_SCRIPT.

Change-Id: I993f12766c96274f096c5f6c6da5aaa32394abbc