Fix compilation with RC_Channels not available

[peterbarker]

... primarily to easy PERIPH_HEAVY out of the code

[peterbarker]

Board               AP_Periph  blimp  bootloader  copter  heli  iofirmware  plane  rover  sub
Durandal                       *      *           *       *                 *      *      *
HerePro             *                                                           
Hitec-Airspeed      *                                                           
KakuteH7-bdshot                *      *           *       *                 *      *      *
MatekF405                      *      *           *       *                 *      *      *
Pixhawk1-1M-bdshot             *                  *       *                 *      *      *
f103-QiotekPeriph   *                                                            
f303-Universal      *                                                            
iomcu                                                           *                
revo-mini                      *      *           *       *                 *      *      *
skyviper-v2450                                    *