AP_Compass: prevent bad initial values from affecting HMC5883 calibration

the first couple of values after we enter strap mode may be low, but
just above our 0.7 threshold. We now discard the first two values to
prevent these affecting the average.

Also added some commented out debug code and a comment on the scaling
of the calibration code
PX4: run hmc5883 compass calibration on every boot

this gives us more consistent compass readings between boots
PX4: added "make px4-cleandep" for faster rebuilding between branches