AP_Notify: disable autobuild of example

this fails due to an AVR linker bug
build: reduced PX4 warnings and avoid rebuilding when possible
Copter: fixed some build warnings
AC_WPNav: fixed some build warnings
AP_Airspeed: fixed a build warning
AP_GPS: fixed a build warning
HAL_PX4: removed an unused variable
AP_Math: added M_PI_F define
PX4: added -Wno-packed

we know that packed data structures are not as efficient
AP_InertialSensor: make sure we wait for a sample before update()
AP_InertialSensor: fixed loop limits

stops pointless operations
AP_AHRS: Added getters for compass and GPS.
AP_Arming: First commit of AP_Arming library.

Intended to prevent unintentional motor startups.
Plane: Now using AP_Arming library.
Plane: when no GPS lock always send GPS messages
AP_Arming: fixed build
Plane: fixed arming bit in MAVLink heartbeat message
AP_Arming: fixed public/private split
Plane: small arming tidyups
AP_Arming: make it possible to change ARMING_REQUIRED without a reboot