Copter: consider health of primary compass in SYS_STATUS, pre-arm and compassmot

this should prevent users from taking off with only secondary compass
AP_NavEKF : Clean up angle and mag field initialisation

This path reduces duplicated code, eliminates unused variables and
causes the earth magnetic field states to be reset when exiting static mode
which will occur every time copter is armed. This enables copters to be
powered on and initialised inside vehicles or houses, without bad earth
field values affecting flight.
AP_NavEKF : Reduce GPS position innovation consistency fail threshold

This patch reduces the maximum acceptable GPS jump from approximately 16 to 8 metres
This will provide copters with more protection for close in loiter situations