Allow to call dist-for-goals multiple times for the same file

You can dist the same file for multiple goals in multiple calls to
dist-for-goals. The first call will establish the real copy rule, while
the rest call just establishes the goals' dependency on the dest file.
This enable uss to remove the bizarre $(if ..) enclosing the droid and
sdk dist while avoiding make's multiple rules warning.

Change-Id: I76475db76a9e6167e0e606dd582b54e80dfcdd22