kernel-install: Remove existing initramfs if it's older than the kernel
    
    Currently we fail to install a new kernel with "make install" on the second
    try, because dracut won't over-write the initramfs without --force.  In this
    patch, 20-grub.install will remove an existing initramfs if it's newer than the
    kernel we're installing.
    
    Resolves: rhbz#1638405
    Signed-off-by: Peter Jones <pjones@redhat.com>