bitbake: Fix try_mirrors() logic, was only trying the first mirror.

The logic in try_mirrors() was buggy such that only the first entry in the
mirrors list was tried.

This patch fixes this. Thanks to Richard for the review.

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

bitbake/lib/bb/fetch/__init__.py

@@ -383,7 +383,8 @@ def try_mirrors(d, uri, mirrors):
                 import sys
                 (type, value, traceback) = sys.exc_info()
                 bb.msg.debug(2, bb.msg.domain.Fetcher, "Mirror fetch failure: %s" % value)
-                return ""
+                continue
+    return None
 
 
 class FetchData(object):