General cleanup of /dev/px4io and /dev/px4fmu

- Use distinct common symbols for px4io and px4fmu device files, and use
instead of hardcoded filenames
- Use common symbols defining px4io bits consistently between px4fmu and
px4io builds.

src/drivers/px4fmu/fmu.cpp

@@ -166,7 +166,7 @@ PX4FMU	*g_fmu;
 } // namespace
 
 PX4FMU::PX4FMU() :
-	CDev("fmuservo", "/dev/px4fmu"),
+	CDev("fmuservo", PX4FMU_DEVICE_PATH),
 	_mode(MODE_NONE),
 	_pwm_default_rate(50),
 	_pwm_alt_rate(50),