Flight rec data dump with gdb script

This adds a gdb script and a global variable that points the ring buffer as to display the contents of flight recorder in case of a core-dump.

A continuation for !144 (comment 145935)

Once in gdb source the script and call 'display_flight_rec' to dump the data.

The symbol must be present, initialized and not empty in order to display its contents. The data can also be accessed calling the function weston_log_flight_recorder_display_buffer but this can't be used in core-dump situation only when the program is a running state.