The curly brace was in the wrong spot.

This reverts commit 71b2306a.

This broke the build due to git submodules. We need to work out how
ArduPilot is going to handle the external uavcan module dependency
before we enable this again

GNSS modules handled by PX4 drivers are not auto-detectable, some are not even
connected to a UART port. The activation is therefore controlled by GPS_TYPE
only. Baud rate and port settings (if applicable) have to be handled by the PX4
firmware.

Using a simple bit mask to avoid calculating an unneeded ATAN2() for AVR users.

The modification allows the read and write functions to be called by any thread but the calling thread must be the last one that called the begin() function.

Fixes #1321

- rc_override_active is never set anywhere in the ArduPlane code; it's only used for Copter and Rover. Removing it significantly simplifies the failsafe code.
- modified code has been tested in SITL. Heartbeat and RC failures in AUTO, CRUISE, and RTL modes (covering the three cases in the failsafe check functions) have been simulated with FS_LONG_ACTN = 0, 1, and 2, FS_SHORT_ACTN = 0, 1, and 2, and FS_GCS_ENABL = 0, 1, and 2. In all cases the results are identical to those with the original code.

As suggested in the thread at the following link:
https://groups.google.com/forum/#!topic/drones-discuss/hiD23c3w6xQ

(Plane Only) If the yaw and GPS heading disagree by more than 45 degrees on takeoff, then the magnetometer is declared as failed. The heading is then reset based on the difference between GPS ground track and stgate velocity vector.
Magnetometer fusion uses corrected data and bias states are initialised to zero. This allows the compass to be switched in flight.
For persistent compass errors that trigger a timeout, the compass is not permanently failed, however for non-forward fly vehicles the compass weighting is reduced.

exit from the autpilot when reboot is commanded.
The software assumes that the code is being
launched in an infinite loop thereby an exit
will make it reboot.