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/device.cpp

@@ -42,6 +42,7 @@
 #include <nuttx/arch.h>
 #include <stdio.h>
 #include <unistd.h>
+#include <drivers/drv_device.h>
 
 namespace device
 {
@@ -93,6 +94,13 @@ Device::Device(const char *name,
 	_irq_attached(false)
 {
 	sem_init(&_lock, 0, 1);
+        
+	/* setup a default device ID. When bus_type is UNKNOWN the
+	   other fields are invalid */
+	_device_id.devid_s.bus_type = DeviceBusType_UNKNOWN;
+	_device_id.devid_s.bus = 0;
+	_device_id.devid_s.address = 0;
+	_device_id.devid_s.devtype = 0;
 }
 
 Device::~Device()
@@ -238,6 +246,10 @@ Device::write(unsigned offset, void *data, unsigned count)
 int
 Device::ioctl(unsigned operation, unsigned &arg)
 {
+	switch (operation) {
+	case DEVIOCGDEVICEID:
+		return (int)_device_id.devid;
+	}
 	return -ENODEV;
 }