[radio control] spektrum also usable for intermcu

Alternative to #843

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`.
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.