The fetcher has special handling for file:// mirror urls, being efficient and
just providing an updated path. Unfortunately the sstate fetching code wasn't
able to handle this. This patch detects this and injects a symlink to ensure
everything works. It also fixes some datastore references to be correct and
ensures the sstate download directory exists if it doesn't already.
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>
