Tasks for TFTP upload

This is to change the existing TFTP update into an asynchronous service
that uses Redfish Tasks.

Tested:

  TFTP Firmware update through redfish,and get the task to create.

  curl -k -H "X-Auth-Token: $token" -X POST
  https://${bmc}/redfish/v1/UpdateService/Actions/UpdateService.SimpleUpdate
  -d '{"TransferProtocol":"TFTP","ImageURI":"xx.xx.xx.xx/obmc-phosphor-xxxxx"}'
  {
    "@odata.id": "/redfish/v1/TaskService/Tasks/0",
    "@odata.type": "#Task.v1_4_3.Task",
    "Id": "0",
    "TaskState": "Running",
    "TaskStatus": "OK"
  }

Signed-off-by: Albert Zhang <zhanghaodi@inspur.com>
Change-Id: I5d8683f38bf3e29177d90606c5c9fe747a26a876