PX4: enable ccache on windows build
AP_NavEKF: Reduce vulnerability to ground fixed magnetic interference

Re-initialisation of the magnetic field states and yaw angle is now only performed a maximum of two times after start-up.

Once when coming out of static modefor the first time (first arm event)
Again (for copter only) when the altitude gain above the arming altitude exceeds 1.5m

this prevents magnetic interference present at arming (eg arming on a metal roof)from corrupting the magnetic field states enough to cause bad heading errors and toilet bowling on copter
Copter: run_nav_updates at 100hz on pixhawk
Copter: run_nav_updates at 50hz on Pixhawk, 25hz on APM2

Based on work by Jon Challinger (see earlier commit)