AP_BoardConfig: work around issues with bad components on some FMUv4 boards

some pixracer copies have poor components which can cause sensor bus
brownout on boot. This resets the sensor bus for 20ms on boot to try
to get clean sensor startup

libraries/AP_BoardConfig/px4_drivers.cpp

@@ -51,6 +51,7 @@ extern "C" {
     int mb12xx_main(int, char **);
     int pwm_input_main(int, char **);
     int uavcan_main(int, char **);
+    int fmu_main(int, char **);
 };
 
 
@@ -468,6 +469,17 @@ void AP_BoardConfig::px4_start_fmuv4_sensors(void)
  */
 void AP_BoardConfig::px4_start_common_sensors(void)
 {
+#if defined(CONFIG_ARCH_BOARD_PX4FMU_V4)
+    /*
+      this works around an issue with some FMUv4 hardware (eg. copies
+      of the Pixracer) which have incorrect components leading to
+      sensor brownout on boot
+     */
+    if (px4_start_driver(fmu_main, "fmu", "sensor_reset 20")) {
+        printf("FMUv4 sensor reset complete\n");        
+    }
+#endif
+
     if (px4_start_driver(ms5611_main, "ms5611", "start")) {
         printf("ms5611 started OK\n");
     } else {