bitbake: if PREMIRRORS set test for local file in FetchData.setup_localpath

bitbake: if PREMIRRORS set test for local file in FetchData.setup_localpath

When we are using PREMIRRORS it's possible a mirror in the local namespace
(some filesystem path, i.e. an NFS share) provides read-only files.
This is a perfectly valid scenario so this patch fixes bitbake so that for
such a scenario locapath is set to the files path rather than some child
of DL_DIR.

Signed-off-by: Joshua Lock <josh@linux.intel.com>