Added hardware flow control to mavlink app. Auto-disables if nothing can be written to the port
Fix FIONWRITE usage
Merge branch 'beta_mavlink2' into beta_mavlink2_flow_control
Fixed HW flow control enable / disable scheme, console output cleanup on start
Fine tuning for flow control disable to stop firing after change is effective
Merge pull request #727 from PX4/beta_mavlink2_flow_control

UART flow control