ekf2: make filter update period configurable

- introduces new parameter EKF2_PREDICT_US to configure the filter
update period in microseconds
 - actual filter update period will be an integer multiple of IMU

src/modules/ekf2/ekf2_params.c

@@ -39,6 +39,19 @@
  *
  */
 
+/**
+ * EKF prediction period
+ *
+ * EKF prediction period in microseconds. This should ideally be an integer multiple of the IMU time delta.
+ * Actual filter update will be an integer multiple of IMU update.
+ *
+ * @group EKF2
+ * @min 1000
+ * @max 20000
+ * @unit us
+ */
+PARAM_DEFINE_INT32(EKF2_PREDICT_US, 10000);
+
 /**
  * Magnetometer measurement delay relative to IMU measurements
  *