allow MAG_ENABLE to be changed in flight

this disables the compass in DCM if MAG_ENABLE is changed in
flight. Without this we would use a fixed yaw once the compass is
disabled

This also makes sure we don't pass the compass to DCM till we have
done a read. This ensures we have a good compass fix for the initial
DCM heading

ArduCopter/sensors.pde

@@ -78,7 +78,7 @@ static int32_t read_barometer(void)
 static void init_compass()
 {
 	compass.set_orientation(MAG_ORIENTATION);						// set compass's orientation on aircraft
-	if (!compass.init()) {
+	if (!compass.init() || !compass.read()) {
         // make sure we don't pass a broken compass to DCM
         Serial.println_P(PSTR("COMPASS INIT ERROR"));
         return;

ArduPlane/ArduPlane.pde

@@ -769,8 +769,11 @@ static void medium_loop()
 
 			#if HIL_MODE != HIL_MODE_ATTITUDE
             if (g.compass_enabled && compass.read()) {
+                dcm.set_compass(&compass);
                 compass.calculate(dcm.get_dcm_matrix());  // Calculate heading
                 compass.null_offsets(dcm.get_dcm_matrix());
+            } else {
+                dcm.set_compass(NULL);
             }
 			#endif
 /*{

ArduPlane/system.pde

@@ -175,12 +175,11 @@ static void init_ardupilot()
 
 	if (g.compass_enabled==true) {
         compass.set_orientation(MAG_ORIENTATION);							// set compass's orientation on aircraft
-		if (!compass.init()) {
+		if (!compass.init() || !compass.read()) {
             Serial.println_P(PSTR("Compass initialisation failed!"));
             g.compass_enabled = false;
         } else {
             dcm.set_compass(&compass);
-            compass.get_offsets();						// load offsets to account for airframe magnetic interference
             compass.null_offsets_enable();
         }
 	}