Copter: Add using_interlock flag, employ it in arming process

ArduCopter/AP_State.pde

@@ -146,3 +146,10 @@ void set_motor_interlock(bool b)
     }
 }
 
+void set_using_interlock(bool b)
+{
+    if(ap.using_interlock != b) {
+        ap.using_interlock = b;
+    }
+}
+

ArduCopter/ArduCopter.pde

@@ -358,6 +358,7 @@ static union {
         uint8_t gps_base_pos_set    : 1; // 29      // true when the gps base position has been set (used for RTK gps only)
         enum HomeState home_state   : 2; // 30,31   // home status (unset, set, locked)
         uint8_t motor_interlock     : 1; // 32      // motor interlock status, final control for motors on/off
+        uint8_t using_interlock     : 1; // 33      // aux switch motor interlock function is in use
     };
     uint32_t value;
 } ap;

ArduCopter/motors.pde

@@ -157,6 +157,16 @@ static bool init_arm_motors(bool arming_from_gcs)
         did_ground_start = true;
     }
 
+    // check if we are using motor interlock control on an aux switch
+    set_using_interlock(check_if_auxsw_mode_used(AUXSW_MOTOR_INTERLOCK));
+
+    // if we are using motor interlock switch and it's disabled, fail to arm
+    if (ap.using_interlock && ap.motor_interlock){
+        gcs_send_text_P(SEVERITY_HIGH,PSTR("Arm: Motor Interlock Enabled"));
+        AP_Notify::flags.armed = false;
+        return false;
+    }
+
     // enable gps velocity based centrefugal force compensation
     ahrs.set_correct_centrifugal(true);
     hal.util->set_soft_armed(true);
@@ -206,6 +216,18 @@ static bool pre_arm_checks(bool display_failure)
         return true;
     }
 
+    // check if motor interlock aux switch is in use
+    // if it is, switch needs to be in disabled position to arm
+    // otherwise exit immediately.  This check to be repeated, 
+    // as state can change at any time.
+    set_using_interlock(check_if_auxsw_mode_used(AUXSW_MOTOR_INTERLOCK));
+    if (ap.using_interlock && ap.motor_interlock){
+        if (display_failure) {
+            gcs_send_text_P(SEVERITY_HIGH,PSTR("PreArm: Motor Interlock Enabled"));
+        }
+        return false;
+    }
+
     // exit immediately if we've already successfully performed the pre-arm check
     if (ap.pre_arm_check) {
         // run gps checks because results may change and affect LED colour