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