[radio control] spektrum also usable for intermcu
Merge pull request #847 from paparazzi/intermcu_rc

[radio control] spektrum also usable for intermcu

You have to add the radio_control subsystem to the ap target as well in order to get the RADIO_CONTROL_TYPE_H define to load the correct implementation header so we get the RADIO_CONTROL_NB_CHANNEL and channel names.
For spektrum you might have to define RADIO_CONTROL_NB_CHANNEL in your airframe file to something less than 10 to use it with the uart inter_mcu.