bitbake: fetch2/__init__.py: let try_mirror_url return correct value

The fetcher will try:

1) PREMIRROR
2) Upstream
3) MIRROR

If it fails to download from the Upstream, but succeeds from the MIRROR,
and ud.localpath != origud.localpath (for example, the git tarball),
then we will get the error (e.g.: xf86-video-omapfb):

ERROR: Function failed: Fetcher failure for URL: 'xxx'. Unable to fetch URL from any source.
ERROR: Logfile of failure stored in: /path/to/log.do_fetch.28024

It should not show the error and let the build go on since it succeeds.
(e.g.: xf86-video-omapfb)

[YOCTO #5686]

(Bitbake rev: 3bb3f1823bdd46ab34577d43f1e39046a32bca77)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/fetch2/__init__.py

@@ -809,7 +809,7 @@ def try_mirror_url(newuri, origud, ud, ld, check = False):
                 origud.method.download(origud.url, origud, ld)
                 if hasattr(origud.method,"build_mirror_data"):
                     origud.method.build_mirror_data(origud.url, origud, ld)
-            return None
+            return ud.localpath
         # Otherwise the result is a local file:// and we symlink to it
         if not os.path.exists(origud.localpath):
             if os.path.islink(origud.localpath):