[radio_control] cleanup channel defines

- PPM: remove PPM_NB_CHANNEL define an always decode RADIO_CTL_NB (number of channels in radio.xml)
- PPM: the actually used/needed channels RADIO_CONTROL_NB_CHANNEL default to RADIO_CTL_NB but can be overwritten by user
  e.g. to use less channels for intermcu
- SBUS: only provide a default for RADIO_CONTROL_NB_CHANNEL
[radio_control] spektrum: copy only actually used channels to radio_control.values
[radio_control] error if RADIO_CONTROL_NB_CHANNEL > max available channels
[conf] Example that needs the RADIO_CONTROL_NB_CHANNEL
[stm32] spektrum: always decode up to 12 channels

but only copy RADIO_CONTROL_NB_CHANNEL to radio_control.values
Merge pull request #975 from paparazzi/cleanup_rc_nb_channel_defines

[radio_control] cleanup channel defines

- PPM: remove PPM_NB_CHANNEL define an always decode RADIO_CTL_NB (number of channels in radio.xml)
- PPM: the actually used/needed channels RADIO_CONTROL_NB_CHANNEL default to RADIO_CTL_NB but can be overwritten by user
  e.g. to use less channels for intermcu
- SBUS: only provide a default for RADIO_CONTROL_NB_CHANNEL
- Spektrum: always try to decode up to SPEKTRUM_NB_CHANNEL (12), but copy only used RADIO_CONTROL_NB_CHANNEL to radio_control.values
- error if RADIO_CONTROL_NB_CHANNEL > max available channels

As far as I can discern this should finally make it possible to override the actually used RADIO_CONTROL_NB_CHANNEL from the airframe file (e.g. to transfer less via intermcu) while still using the proper number of channels for parsing.