Initial support for Apertis hawkBit agent

These commits add:

• configuration support through keyfile
• feedback message support for Deployment Base object
• Config Data message support
• poll of update server (using "sleep time" provided by server)
• trigger Apertis Update Manager to apply update after download finishes
• systemd service file
• Debian package files

data/apertis-hawkbit-agent.ini

+[server]
+base-uri=http://localhost:8080
+#tenant=DEFAULT
+#controller-id=<ControllerID>
+#key-token=<DeviceToken>