APM_OBC: added AFS_ENABLE parameter

AFS system is disabled by default
Plane: build AFS (formerly known as OBC) failsafe on Pixhawk

this gives an advanced failsafe system which may be useful for other
than OBC use
AP_InertialSensor: fixed a build warning
GCS_MAVLink: added send_statustext_all() method

used for sending text messages from libraries
APM_OBC: use send_statustext_all()
Plane: fixed build warning