While and until loops support to GRUB script.

	While and until loops support to GRUB script.

	* include/grub/script_sh.h (grub_script_cmdwhile): New struct.
	(grub_script_create_cmdwhile): New function prototype.
	(grub_script_execute_cmdwhile): New function prototype.
	* script/execute.c (grub_script_execute_cmdwhile): New function.
	* script/parser.y (command): New commands.
	(whilecmd): New grammar rule.
	(untilcmd): New grammar rule.
	* script/script.c (grub_script_create_cmdwhile): New function.
	* util/grub-script-check.c (grub_script_execute_cmdwhile): New
	function.

	* tests/grub_script_while1.in: New testcase.
	* conf/tests.rmk: Rule for new testcase.