AP_GPS: make the examples compile in Linux
HAL_Linux:Create subclass to handle FRAM Storage under main Storage Class
HAL_Linux: Add Storage.cpp containing common storage class member definitions
HAL_Linux: fixed some warnings
HAL_Linux: re-fix dirty mask bug in Storage

when conversion to a single parent class was done it re-introduced a
bug that was fixed a while ago

See commit d238ff7c5dbeb37657cf5980bcbe29fc2c82ffa2
AC_AttControl: remove unused logging structure
AP_MotorsSingle: Fix parameter comments.
Compass: shorten EXTERNAL2/3 to EXTERN2/3

Bug discovered by Michael Day
Copter: CPU failsafe logs an error
Copter: CPU failsafe set motors to min before logging error

This ensures that if the logging stalls the motors will have already
been reduced to zero
Plane: move delay callback setup to after compass and airspeed config

this ensure parameters sent on startup include compass settings and
airspeed offset
Plane: fixed number of channels we pass to PWM limits
Plane: fixed potential division by zero in mixing creation

also setup failsafe values if already armed, so while loading new
mixer the throttle output doesn't go to zero
Plane: update OVERRIDE_CHAN docs
Plane: change default autotune level to 6
AP_HAL_AVR_SITL: fill in ublox accuracy metrics with reasonable values