Allow SRXL RC Protocol to be compiled out

[peterbarker]

Saves 888 bytes when compiled out on CubeOrange

Board              AP_Periph  blimp  bootloader  copter  heli  iofirmware  plane  rover  sub
Durandal                      *      *           *       *                 *      *      *
Hitec-Airspeed     *                                                                     
KakuteH7-bdshot               *      *           *       *                 *      *      *
MatekF405                     *      *           *       *                 *      *      *
Pixhawk1-1M                   *      *           *       *                 *      *      *
f103-QiotekPeriph  *                                                                     
f303-Universal     *                                                                     
iomcu                                                          *                         
revo-mini                     *      *           *       *                 *      *      *
skyviper-v2450                                   -776                                    

... apparently nobody really uses this so cutting it on minimized boards might be an option