https://github.com/diydrones/apm_planner/issues/788

- select active vehicle
- arm/disarm
- select flight mode

Previous code was horribly wrong.

- Also added new adjustMavlinkMessage support

- FirmwarePlugin is the new place for code which is specific to
firmware flight stack
- FirmwarePlugin currently supports flight mode apis
- Modify code to use new FirmwarePlugin support