add overlay support
A user can now define OVERLAYS=path1 path2
The ports tree will lookup into path1 and path2 (the order matters) to find the
path for the port (testing origin/Makefile)
If also have a naive implementation for USES, so one can overwrite existing USES.
Note that for the USES that includes other uses, or will have to also converted
to overlay to be able to properly work