AP_Compass: AK8963: be agnostic to I2C bus/address

This decision is better made by the caller rather than polluting the
driver with board-specific details.

libraries/AP_Compass/AP_Compass_AK8963.cpp

@@ -79,10 +79,6 @@
 #endif
 #endif
 
-#if !defined(HAL_COMPASS_AK8963_I2C_ADDR)
-#define HAL_COMPASS_AK8963_I2C_ADDR 0xC
-#endif
-
 extern const AP_HAL::HAL& hal;
 
 AP_Compass_AK8963::AP_Compass_AK8963(Compass &compass, AP_AK8963_SerialBus *bus) :
@@ -112,11 +108,13 @@ AP_Compass_Backend *AP_Compass_AK8963::detect_mpu9250(Compass &compass)
     return sensor;
 }
 
-AP_Compass_Backend *AP_Compass_AK8963::detect_i2c1(Compass &compass)
+
+AP_Compass_Backend *AP_Compass_AK8963::detect_i2c(Compass &compass,
+                                                  AP_HAL::I2CDriver *i2c,
+                                                  uint8_t addr)
 {
-    AP_Compass_AK8963 *sensor = new AP_Compass_AK8963(compass,
-                                                  new AP_AK8963_SerialBus_I2C(
-                                                  hal.i2c1, HAL_COMPASS_AK8963_I2C_ADDR));
+    AP_Compass_AK8963 *sensor =
+        new AP_Compass_AK8963(compass, new AP_AK8963_SerialBus_I2C(i2c, addr));
 
     if (sensor == nullptr) {
         return nullptr;

libraries/AP_Compass/AP_Compass_AK8963.h

@@ -37,7 +37,9 @@ public:
     AP_Compass_AK8963(Compass &compass, AP_AK8963_SerialBus *bus);
 
     static AP_Compass_Backend *detect_mpu9250(Compass &compass);
-    static AP_Compass_Backend *detect_i2c1(Compass &compass);
+    static AP_Compass_Backend *detect_i2c(Compass &compass,
+                                          AP_HAL::I2CDriver *i2c,
+                                          uint8_t addr);
 
     bool        init(void);
     void        read(void);

libraries/AP_Compass/Compass.cpp

@@ -348,7 +348,8 @@ void Compass::_detect_backends(void)
 #elif HAL_COMPASS_DEFAULT == HAL_COMPASS_HMC5843
     _add_backend(AP_Compass_HMC5843::detect(*this));
 #elif  HAL_COMPASS_DEFAULT == HAL_COMPASS_AK8963_I2C && HAL_INS_AK8963_I2C_BUS == 1
-    _add_backend(AP_Compass_AK8963::detect_i2c1(*this));
+    _add_backend(AP_Compass_AK8963::detect_i2c(*this, hal.i2c1,
+                                               HAL_COMPASS_AK8963_I2C_ADDR));
 #elif HAL_COMPASS_DEFAULT == HAL_COMPASS_PX4 || HAL_COMPASS_DEFAULT == HAL_COMPASS_VRBRAIN
     _add_backend(AP_Compass_PX4::detect(*this));
 #else