APM: added the ability to forcibly crash the plane on OBC failsafe breach

this allows a user to setup the OBC failsafe system to forcibly crash
the plane (surfaces at limits, zero throttle) when the failsafe system
triggers. This is to allow APM to be used in the Outback Challenge. In
the OBC an external failsafe board also does this using the heartbeat
control pin, so this is an extra safety mechanism.

To prevent users accidentially triggering a crash, this code only
activates if FS_TERM_ACTION is set to to the magic value 42.