Allow Periph to compile with !AP_GPS_ENABLED

[peterbarker]

No description provided.

[peterbarker]

I've run this through size-compare-branches.py and it's all-stars except Durandal (due to some optimisation things in AP_AHRS.cpp), and HerePro which now gets AP_AHRS_ENABLED.

HerePro has scripting enabled, that has methods in it into the Location library which require AP_AHRS - so we need to compile the AP_AHRS library in. It will still die if it ever calls the AHRS methods as we won't be instating the singleton.

Can't use PH's new per-method depends as it doesn't seem to work on userdata things.

[peterbarker]

... added a patch that creates the AHRS singleton if AP_AHRS_ENABLED is true, which would stop HerePro from dying if you were to call AHRS methods on it.

CubeOrange-Periph-heavy got 880 bytes smaller as it doesn't include scripting and thus no AHRS is required.