AP_OAPathPlanner: using static constexpr instead const

[peterbarker]

Extracted from d6a0fbe so we can discuss the meat of (and rename) that other PR properly.

@xianglunkai

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

[Ryanf55]

While switching from #define to static const makes sense for compile time size checks, what does this switch buy us?

[peterbarker]

While switching from #define to static const makes sense for compile time size checks, what does this switch buy us?

Simplifies the other PR.

That's all I was after....