grubby-bls: only compare using relative paths if /boot is a mount point
    
    The grub2 bootloader expects the BLS linux and initrd fields values to be
    set to a relative path to the root of the boot partition and the zipl tool
    expects these to be an absolute path to the kernel and initramfs images.
    
    So the grubby wrapper was removing the prefixes from the kernel and initrd
    paths before doing any comparision with the BLS fields. But this shouldn't
    be done if the /boot directory isn't a mount point.
    
    Resolves: rhbz#1642078
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>