Copter: OLED display always says EKF:ok

It seems the "EKF: ok" message is always displayed even if the EKF isn't healthy. To reproduce the problem:

- attach an oled display to a flight controller which has an external compass
- rotate the external compass until the ekf goes red
- note the "EKF: ok" remains

My guess is that only the case of an EKF failsafe does the EKF go bad. But we currently don't allow an EKF failsafe to trigger while we're disarmed so this will never be seen.

This issue was raised on AC3.5 beta testing thread and confirmed by me (randy): http://discuss.ardupilot.org/t/oled-display-not-update-ekf/17954/4

oled wiki page: http://ardupilot.org/copter/docs/common-display-onboard.html

Reproduce the issue using the Copter OLED display, an external compass, and a flight controller: rotate the compass until the EKF goes red and observe the displayed status. Compare the OLED's EKF indication with the actual EKF health, including the disarmed state; done means the display no longer reports "EKF: ok" when the EKF is unhealthy.

Written by the indexing model from the issue text.