EKF: Don't start the output observer before the main filter has initialised

This prevents the possibility of output transients if alignment is delayed.

EKF/ekf.cpp

@@ -160,8 +160,6 @@ bool Ekf::init(uint64_t timestamp)
 bool Ekf::update()
 {
 
-	// Only run the filter if IMU data in the buffer has been updated
-	if (_imu_updated) {
 		if (!_filter_initialised) {
 			_filter_initialised = initialiseFilter();
 
@@ -170,6 +168,9 @@ bool Ekf::update()
 			}
 		}
 
+	// Only run the filter if IMU data in the buffer has been updated
+	if (_imu_updated) {
+
 		// perform state and covariance prediction for the main filter
 		predictState();
 		predictCovariance();