AP_InertialSensor: make PX4 healthy call _get_sample()

this prevents a false positive during times like arming where we are
not reading the sensors
Copter: force gyro calibration on start-up
Copter: GPS ok only when not glitching

Removed redundant checks to GPS_ok before setting flight mode to RTL
(this check is already performed inside the set_mode function)
Removed reset of home distance and bearing when GPS lock is lost, it now
remains at the last known value
Copter: bug fix to take-off in Loiter and AltHold

Always set the target altitude after take-off to be current altitude +
20cm.  This resolves a bug in which the target altitude could end up
being a couple of meters higher than the current altitude if the user
entered Loiter / Alt Hold mode before inertial nav altitude estimate had
settled.
Copter: gps failsafe disabled until we get first 3d lock
Iris: remove ATTTITUDE_FAST, PID and OPTFLOW logging