grubby-bls: strip only /boot from paths
    
    If the /boot directory is a mount point, then the paths of the kernel and
    initramfs images paths in the BLS snippet have to be relative to the root
    of the boot partition. So the /boot prefix has to be stripped from paths
    in this case.
    
    But the function was wrongly stripping the entire path instead of only the
    /boot prefix, which prevented having kernels and initrds in subdirectories.
    
    Signed-off-by: Yuval Turgeman <yturgema@redhat.com>