update_layer: fix tinfoil shutdown

Fix a couple of problems with shutting down tinfoil:

1) tinfoil.shutdown() wasn't being called on error because it wasn't
   inside a finally: section, thus on error the script hung with bitbake
   master (since the tinfoil2 changes) - this is almost certainly a bug
   in bitbake but let's handle it here anyway.
2) We need to check whether the tinfoil object actually has a shutdown
   attribute since we still want to support updating for older branches
   where tinfoil didn't have a shutdown() method.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>

layerindex/update_layer.py

@@ -678,8 +678,10 @@ def main():
     except:
         import traceback
         traceback.print_exc()
+    finally:
+        if hasattr(tinfoil, 'shutdown'):
+            tinfoil.shutdown()
 
-    tinfoil.shutdown()
     shutil.rmtree(tempdir)
     sys.exit(0)