Allow system configs to be read from vendor partition

Soc vendors also want to add their own configs like odms do.
Additionally they should be allowed to add their own app permission
configs because they can install their own apps in /vendor/app.
So Soc vendors should be able to add system configs around libs,
features, permissions and apps.

Additionally this CL modified codes to allow "privapp-permissions"
only on system partition because we won't allow apps on the partner
partitions to count as privileged.

Test: building succeeded and tested on sailfish.
Bug: 35369237
Change-Id: I7d84d6e351d9e7023931757082d9f661c5a9a80a