20-grub-install: Replace, rather than overwrite, the existing kernel.
    
    In rhbz#1638405, we worked around the issue of an existing initramfs
    being in the way by removing it if it's older than the kernel we're in
    the process of installing.
    
    But it was buggy and only worked with some filesystem layouts and paths, and
    also possibly had some issues with file creation times causing the shell -ot
    comparison to fail in some cases.
    
    This patch changes it to remove the existing kernel (as well as other
    related files) in the case that it's going to do the copy, and also fixes the
    path issues.
    
    Resolves: rhbz#1642402
    Related: rhbz#1638405
    
    Signed-off-by: Peter Jones <pjones@redhat.com>
    Tested-by: Prarit Bhargava <prarit@redhat.com>