Rename sum8 to sum8_with_carry

[peterbarker]

@Ryanf55 pointed out that "sum8" should be reserved for sum-bytes-modulus-256 only as that's the common name for that algorithm.

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                                    *