HAL_PX4: fixed delay_microseconds() on PX4

we could underflow and delay a very long time!
AP_InertialSensor: added wait_for_sample() API call

this waits for a new INS sample to arrive, using whatever method is
most efficient on each INS type
Plane: use ins.wait_for_sample()
Copter: use ins.wait_for_sample() for main loop

this takes advantage of the INS specific method to wait for a sample
Rover: use ins.wait_for_sample()
AP_InertialSensor_L3G4200D: a sample is only available if gyro had a sample
AP_InertialSensor: use gyro sample count on L3G4200D

this gives more even timing in ArduCopter
Rover: enable logging on Linux
Copter: enable logging on Linux
Copter: update firmware version to AC3.1-rc3
GPSGlitch: parameters for accel and always-ok-radius
AutoTest: slightly shorten copter loiter test
AutoTune: abandon tuning if rate P hits minimum

Fixes from Leonard
Copter: pre-arm expects mag len of 330 for APM1 (was 530)