AP_HAL: added get_flow_control() API

libraries/AP_HAL/UARTDriver.h

@@ -43,6 +43,7 @@ public:
         FLOW_CONTROL_DISABLE=0, FLOW_CONTROL_ENABLE=1, FLOW_CONTROL_AUTO=2
     };
     virtual void set_flow_control(enum flow_control flow_control_setting) {};
+    virtual enum flow_control get_flow_control(void) { return FLOW_CONTROL_DISABLE; };
 
     /* Implementations of BetterStream virtual methods. These are
      * provided by AP_HAL to ensure consistency between ports to

libraries/AP_HAL_PX4/UARTDriver.h

@@ -37,6 +37,7 @@ public:
     }
 
     void set_flow_control(enum flow_control flow_control);
+    enum flow_control get_flow_control(void) { return _flow_control; }
 
 private:
     const char *_devpath;