20-grub.install: Copy device-tree directory recursively
    
    8800efcb0bea replaced '-a' with '--preserve=timestamps' to avoid
    preserving ownership information on non vfat file systems. This breaks
    copying of the 'dtb' directory on aarch64 systems since '-a' implies
    '-r'. Add '-r' to the single place where 'dtb/' is copied to /boot.
    
    Resolves: #2243060
    Fixes: 8800efcb0bea ("Do not preserve ownership or xattrs on copied files")
    Signed-off-by: Janne Grunau <j@jannau.net>
    Signed-off-by: Nicolas Frayer <nfrayer@redhat.com>