Make grubby to also update GRUB_CMDLINE_LINUX in /etc/default/grub
    
    The grubby --update-kernel=ALL option is used to update the kernel command
    line parameters for all the boot entries. But the updated cmdline is lost
    if the grub2-mkconfig command is executed, since in that case the cmdline
    is changed with the value set by GRUB_CMDLINE_LINUX in /etc/default/grub.
    
    To prevent the cmdline set by users to be overridden by mistake, keep the
    value of GRUB_CMDLINE_LINUX in sync with the current cmdline.
    
    Related: rhbz#1287854
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>