base.bbclass: fix bug with absolute file:// uris

base.bbclass: fix bug with absolute file:// uris

The code which used host/path from the url to construct the destination of the
copy was using os.path.join.  os.path.join treats a "/" in an element as an
indication to drop all previous elements, so the full path resulted in a
destination exactly the same as the source, resulting in a no-op unpack.
Fixed by using oe.path.join instead.

The previous fix missed two instances of os.path.join -- there's code
duplication in oe_unpack_file, really need to resurrect my do_unpack cleanup
branch.

Signed-off-by: Chris Larson <chris_larson@mentor.com>