AP_InertialSensor: identify backend with ID

Identify backend with an id, allowing other libraries to connect to
them. This is different from the _product_id member because it
identifies the sensor, not the board the sensor is in, which is
meaningless for our use case.

libraries/AP_InertialSensor/AP_InertialSensor.cpp

@@ -1,5 +1,7 @@
 /// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: nil -*-
 
+#include <assert.h>
+
 #include <AP_Progmem/AP_Progmem.h>
 #include "AP_InertialSensor.h"
 
@@ -388,6 +390,23 @@ void AP_InertialSensor::_start_backends()
     }
 }
 
+/* Find a backend that has already been succesfully detected */
+AP_InertialSensor_Backend *AP_InertialSensor::_find_backend(int16_t backend_id)
+{
+    assert(_backends_detected);
+
+    for (uint8_t i = 0; i < _backend_count; i++) {
+        int16_t id = _backends[i]->get_id();
+
+        if (id < 0 || id != backend_id)
+            continue;
+
+        return _backends[i];
+    }
+
+    return nullptr;
+}
+
 void
 AP_InertialSensor::init( Start_style style,
                          Sample_rate sample_rate)

libraries/AP_InertialSensor/AP_InertialSensor.h

@@ -244,6 +244,7 @@ private:
     void _add_backend(AP_InertialSensor_Backend *backend);
     void _detect_backends(void);
     void _start_backends();
+    AP_InertialSensor_Backend *_find_backend(int16_t backend_id);
 
     // gyro initialisation
     void _init_gyro();

libraries/AP_InertialSensor/AP_InertialSensor_Backend.h

@@ -63,6 +63,8 @@ public:
      */
     int16_t product_id(void) const { return _product_id; }
 
+    int16_t get_id() const { return _id; }
+
 protected:
     // access to frontend
     AP_InertialSensor &_imu;
@@ -94,6 +96,9 @@ protected:
     // backend should fill in its product ID from AP_PRODUCT_ID_*
     int16_t _product_id;
 
+    // backend unique identifier or -1 if backend doesn't identify itself
+    int16_t _id = -1;
+
     // return the default filter frequency in Hz for the sample rate
     uint8_t _accel_filter_cutoff(void) const { return _imu._accel_filter_cutoff; }