Ensure /system/vendor symlink exists in target-files-package

This used to happen in the system image creation, but the target-files
didn't really depend on the system image, so I removed that dependency.
But then we weren't creating the symlink all the time.

This should be safe to run in parallel with the same link step in the
system creation.

Bug: 36540023
Test: m -j target-files-package, ensure vendor symlink is created
      without system.img.
Change-Id: I37059fa1ce759a0870dc4c65c5515050992245e2