introduce pa_addons instead of mess with packages, vendor_props and manifests
This introduces pa_addons which combines treatment of vendorprops, packages, manifests and per-device repos.

Effetcs:
* By default everything looks and works the same!
* Add/Remove repos should be more error prone.
* Less code duplication.
* More logical structure.
* CM packages pack aligned with CM10.2 and can be enabled by setting EXTRA_CM_PACKAGES to true.

The new structure is:
* For devices:
    vendor/pa/manifests/DEVICE.{addons,removes} (All vendorprop and per-device repos migrated here)
    vendor/pa/packages/DEVICE.mk
    products/pa_DEVICE.mk (All repos/packages migrated to unified vendor/pa/config/pa_addons.mk

* For packages packs (currently only CM)
    vendor/pa/manifests/PACK.{addons,removes}
    vendor/pa/packages/PACK.mk

* Tools:
    tools/{add,remove}repos.py (replaced tools/{getvendorprops,removeprojects,addprojects}.py)

Change-Id: I9a30032dbe003fc14a0df647045c211075b2f5cc