[imu] fix default/weak implementation of imu_periodic

should fix #972

sw/airborne/subsystems/imu.c

@@ -226,6 +226,10 @@ void imu_SetBodyToImuCurrent(float set) {
 
 // weak functions, used if not explicitly provided by implementation
 
+void WEAK imu_scale_gyro(void)
+{
+}
+
 void WEAK imu_scale_gyro(struct Imu* _imu)
 {
   RATES_COPY(_imu->gyro_prev, _imu->gyro);

sw/airborne/subsystems/imu.h

@@ -32,9 +32,10 @@
 #include "math/pprz_orientation_conversion.h"
 #include "generated/airframe.h"
 
-/* must be defined by underlying hardware */
+/** must be defined by underlying hardware */
 extern void imu_impl_init(void);
-extern void WEAK imu_periodic(void); //Optional
+/** optional. If not provided by implementation, empty function is used */
+extern void imu_periodic(void);
 
 /** abstract IMU interface providing fixed point interface  */
 struct Imu {