Only enable PINIOBOX task when needed

Previously the task was always enabled and there's no reason for it to be running if there are no boxID associations.

Saves a few cycles by not running. But has a bigger effect on the scheduler by minimizing the number of active tasks when possible.

src/test/unit/flight_imu_unittest.cc

@@ -249,4 +249,5 @@ bool accGetAccumulationAverage(float *) { return false; }
 void mixerSetThrottleAngleCorrection(int) {};
 bool gpsRescueIsRunning(void) { return false; }
 bool isFixedWing(void) { return false; }
+void pinioBoxTaskControl(void) {}
 }