Only try to generate a config if the ESP is mounted
    
    The posttran scriptlet attempts to generate a GRUB configuration if there
    isn't one in the EFI System Partition. But this leads to a failure if the
    grub2 package is installed in a container.
    
    To avoid this issue, only attempt to generate a GRUB config if the ESP is
    mounted in /boot/efi.
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>