contrib: add early grub config

contrib/image/grub_early.cfg

+if [ -z "$prefix" -o ! -e "$prefix" ]; then
+	if ! search --file --set=root /.disk/info; then
+		search --file --set=root /.disk/mini-info
+	set prefix=($root)/boot/grub
+if [ -e $prefix/x86_64-efi/grub.cfg ]; then
+	source $prefix/x86_64-efi/grub.cfg
+else
+	source $prefix/grub.cfg
+/boot/grub