libreboot-utils: Add cbmem
    
    The cbmem utility enables to retrieve the Coreboot and Libreboot boot
    logs. This requires Coreboot or Libreboot to be built with
    CONFIG_CONSOLE_CBMEM=y to work.
    
    In addition cbmem also supports retrieving additional boot information
    like timestamps (which requires CONFIG_COLLECT_TIMESTAMPS=y), and so
    on.
    
    GRUB also has a cbmem console, so if configured to use it we can also
    retrieve the GRUB boot log.
    
    Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>