OptFlow: fix example sketch so it compiles
Copter: fixed some build warnings
Copter: fence breach causes disarm if landed

Previously it would only disarm if the throttle was also at zero.
Pair programmed with Tridge
Copter: remove unused Log_Write_IMU function
Copter: add DCM_CHECK_THRESH parameter
Copter: add DCM check of yaw error

Triggers an "ekf" failsafe if the DCM yaw error is > 60deg
Copter: enable Optflow by default for Pixhawk only