Make mirror directory if needed.

Signed-off-by: Sébastien Gross <seb•ɑƬ•chezwam•ɖɵʈ•org>
Regenerate grub.cfg before cd building.

Signed-off-by: Sébastien Gross <seb•ɑƬ•chezwam•ɖɵʈ•org>