Rename HAL_PROBE_EXTERNAL_I2C_COMPASSES to AP_COMPASS_PROBING_ENABLED

[peterbarker]

This is a no-compiler-output change.

This PR also force it to have a value, not just be defined.

pbarker@threads:~/rc/ardupilot$ ~/rc/ardupilot/Tools/scripts/filter_size_compare_branches_csv.py --hide-binary-identical /tmp/some.csv  
-----  ---------  -----  ----------  ------  ----  ----------  -----  -----  ---
Board  AP_Periph  blimp  bootloader  copter  heli  iofirmware  plane  rover  sub
-----  ---------  -----  ----------  ------  ----  ----------  -----  -----  ---
pbarker@threads:~/rc/ardupilot$ 

[peterbarker]

(no compiler output change when hwdef changes are excluded from the binary, I should say!)

[peterbarker]

Please can we delay on merging this. This makes all new boards defined in master not work in 4.6 and there are still a few boards in the works for 4.6, I think we should wait a couple more cycles before doing this.

Not repeating the same pain as the LED changes would be nice!

Could you evaluate ~~https://github.com/ArduPilot/ardupilot/compare/master...peterbarker:ardupilot:pr/catch-backport-problem?expand=1~~ https://github.com/ArduPilot/ardupilot/pull/30324 , please - as a patch to go back into 4.6 to avoid backporting errors.

[andyp1per]

Not sure the other patch helps that much.

[tpwrules]

I think the above would be nice but it's not mandatory. I added it to the list and configured all and there were no errors.