Rename SRV_Channel::Aux_servo_function_t to SRV_Channel::Function

[peterbarker]

Not only is the new name shorter, it's also more in-keeping with the way we do enum-class names elsewhere in the code.

Additionally, the word "aux" in the old name is completely misleading - this covers all servo functions, not just "auxiliary" functions!

This is a no-compiler-output-change PR:

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

[rmackay9]

LGTM. As always there are some concerns about difficulties doing backports once this is merged but in that case we could consider backporting this as well I guess

[IamPete1]

Looks good, maybe we can remove the k_ on all the functions in the future too.

[peterbarker]

Looks good, maybe we can remove the k_ on all the functions in the future too.

I did play around with making this actually an enum-class, rather than just looking like one. It involves inserting Function:: into a lot of places where we use this enumeration! Presumably we'd do that at the same to as removing the k_ thing - but you are looking at 1000 patches!

[tridge]

happy with the rename, no enum class change in this PR please

[peterbarker]

I've rebased to resolve conflicts.

Still no compiler output change