Add a sensible default for --config
Add a sensible default for --config

This matches the configuration location that meta-updater uses

Change-Id: Ie96c3462cbbd94781c1e2f0263789aa16f47b352

advancedtelematic/aktualizr,advancedtelematic/sota_client_cpp,advancedtelematic/aktualizr,advancedtelematic/aktualizr,advancedtelematic/sota_client_cpp,advancedtelematic/aktualizr