Add specific defines for sending of GPS mavlink messages

[peterbarker]

This will allow us to remove these messages from the code eventually.

We can also adjust the defines (in a future PR) to remove the ability to send RTK mavlink messages based on which GPS backends are compiled in (only supported on 2 uncommon backends)

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                                         *                                       

[Hwurzburg]

do any need to be included in the build server?

[peterbarker]

do any need to be included in the build server?

Not at this time, I think.

As I mused above, we could potentially remove the RTK sending stuff in a future PR - and then we would need it in the custom build server. Maybe. I'm a bit surprised these aren't gated on moving baseline being compield in or similar, but that might mean the compiler is eliding anyway.

I wanted a no-compiler-output change :-)

[tridge]

I don't think we want or need to be able to control these individually. Instead a single define AP_GPS_LEGACY_MESSAGE_SENDING_ENABLED would be a lot simpler

[peterbarker]

I've pushed up a patch which uses the new more-details defines to save 300-odd bytes on boards that don't have either of ERB or SBF:

Board                    AP_Periph  blimp  bootloader  copter  heli  iofirmware  plane  rover  sub
CubeOrange-periph-heavy  *                                                                     
CubeRedPrimary                      *      *           *       *                 *      *      *
Durandal                            *      *           *       *                 *      *      *
Hitec-Airspeed           *                                                                     
KakuteH7-bdshot                     *      *           *       *                 *      *      *
MatekF405                           -312   *           -312    -312              -312   -320   -312
Pixhawk1-1M-bdshot                  -312               -312    -312              -312   -304   -312
f103-QiotekPeriph        *                                                                     
f303-Universal           *                                                                     
iomcu                                                                *                         
revo-mini                           -312   *           -312    -312              -312   -312   -312
skyviper-journey                                       *                                       
skyviper-v2450                                         -280                                    

[peterbarker]

.... also, if you only have a single GPS (#26751) we can save a few hundred more bytes for a year or two.

too tired