aktualizr-lite: Set a default ECU serial
aktualizr-lite: Set a default ECU serial

You get a warning message every time this tool is run when there is
no ecu serial configured:

  warning: Could not find primary ecu serial, defaulting to empty serial: unknown error

This detects the situation and will give it a "good enough" default.

Signed-off-by: Andy Doan <e3579b1e47f273529f0f929453e939a68ede9fd1@foundries.io>

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