SystemConfig: Allow runtime differentiation of vendor configurations

Single vendor can support multiple sku's with different capabilities.
Add support to load capability xml's from below vendor locations.

vendor/etc/sysconf/sku_${ro.boot.product.vendor.sku}/*.xml
vendor/etc/permissions/sku_${ro.boot.product.vendor.sku}/*.xml

Bug : 148582757
Test: Copy capability xml's to above mentioned vendor locations
      during compilation and check if it gets loaded by
      SystemConfig at runtime.

Change-Id: Ic1a332d30224f6d26afdfc230ea7c1462aefa243