UAS: Add ability to calibrate horizon to level

src/uas/UAS.cc

@@ -1472,6 +1472,9 @@ void UAS::startCalibration(UASInterface::StartCalibrationType calType)
         case StartCalibrationAccel:
             accelCal = 1;
             break;
+        case StartCalibrationLevel:
+            accelCal = 2;
+            break;
     }
     
     mavlink_message_t msg;

src/uas/UASInterface.h

@@ -243,7 +243,8 @@ public:
         StartCalibrationGyro,
         StartCalibrationMag,
         StartCalibrationAirspeed,
-        StartCalibrationAccel
+        StartCalibrationAccel,
+        StartCalibrationLevel
     };
     
     /// Starts the specified calibration