Permit redirection of vendor to system

For devices where /system/vendor needs to be a symlink to /vendor.

This allows a target to set TARGET_COPY_OUT_VENDOR := system in
order to force modules that want to be placed in vendor to instead
be redirected into /system.
eg angler

Change-Id: I4bffcefcda0b33dc5350b1702ec6d0166b18d775