build: Add vendor.prop capability

To get properties on /vendor/build.prop you have to add them via
property overrides in makefiles. This adds a second option,a
vendor.prop file in your device rootdir or TARGET_VENDOR_PROP,
the exact same handling as system.prop.

Change-Id: I8ab2b29bd0b5f0f2bcc0928d6a2a871c40582d3f