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
AP_NavEKF: prevent a possible numerical error on startup

fixes issue #1294
Copter: move relay from ch6 to ch7/ch8
Copter: correct EKF_CHECK parameter description
BaroGlitch: increase max accel to 15m/s/s
Copter: rename EKF check's error message to dataflash

It checks more than the compass so the old error message was misleading.
GCS_MAVLink: added battery2 message
GCS_MAVLink: rebuild MAVLink headers
AP_BattMonitor: added 2nd battery voltage monitoring
GCS_MAVLink: added send_battery2()
Plane: added 2nd battery monitoring support
Plane: move battery init to after parameter load
HAL_PX4: don't report power status change if in first 5 seconds
RC_Channel: fixed k_manual for set_servo_limit()

this allows for failsafe takeover of k_manual channels, which fixes
the behaviour for ignition failsafe
autotest: added -N option to sim_vehicle.sh

useful for unusual builds