srf02 driver: add -b and -a flags to choose i2c busses and remove nuttx stuff such as exit functions

src/drivers/distance_sensor/teraranger/teraranger.cpp

@@ -807,6 +807,11 @@ start_bus(uint8_t rotation, int i2c_bus)
 {
 	int fd = -1;
 
+	if (g_dev != nullptr) {
+		PX4_ERR("already started");
+		return PX4_ERROR;
+	}
+
 	/* create the driver */
 	g_dev = new TERARANGER(rotation, i2c_bus);