Don't write /boot/grub/grub.conf if bootloader is disabled
    
    If user explicitly disabled bootloader then we don't need to write
    /boot/grub/grub.conf. The bootloader is external to disk image (i.e.
    built outside of distribution)
    
    Note:
    - --location=none simply disables bootloader installation
    - --disabled disables bootloader installation and also disables
      installation of the bootloader package
    
    We check for both being present, but we could probably relax it to
    just --disabled.
    
    Signed-off-by: David Abdurachmanov <david.abdurachmanov@gmail.com>