Merge pull request #3323 from PX4/att_est_q_declination

Attitude estimator Q: fix magnetic declination inducing gyro bias growth

src/lib/mathlib/math/Quaternion.hpp

@@ -211,6 +211,16 @@ public:
 		data[3] = static_cast<float>(cosPhi_2 * cosTheta_2 * sinPsi_2 - sinPhi_2 * sinTheta_2 * cosPsi_2);
 	}
 
+	/**
+	 * simplified version of the above method to create quaternion representing rotation only by yaw
+	 */
+	void from_yaw(float yaw) {
+		data[0] = cosf(yaw / 2.0f);
+		data[1] = 0.0f;
+		data[2] = 0.0f;
+		data[3] = sinf(yaw / 2.0f);
+	}
+
 	/**
 	 * create Euler angles vector from the quaternion
 	 */