device: added a _device_id to all drivers

this device ID identifies a specific device via the tuple of (bus, bus
type, address, devtype). This allows device specific configuration
data to be stored along with a device ID, so the code can know when
the user has changed device configuration (such as removing an
external compass), and either invalidate the device configuration or
force the user to re-calibrate

src/drivers/device/i2c.cpp

@@ -62,6 +62,12 @@ I2C::I2C(const char *name,
 	_frequency(frequency),
 	_dev(nullptr)
 {
+	// fill in _device_id fields for a I2C device
+	_device_id.devid_s.bus_type = DeviceBusType_I2C;
+	_device_id.devid_s.bus = bus;
+	_device_id.devid_s.address = address;
+	// devtype needs to be filled in by the driver
+	_device_id.devid_s.devtype = 0;     
 }
 
 I2C::~I2C()
@@ -201,4 +207,4 @@ I2C::transfer(i2c_msg_s *msgv, unsigned msgs)
 	return ret;
 }
 
-} // namespace device
+} // namespace device