kconfig: Don't leak 'option' arguments during parsing

The following strings would leak before this change:

	- option env="LEAKED"
	- option defconfig_list="LEAKED"

These come in the form of T_WORD tokens and are always allocated on the
heap in zconf.l. Free them.

Summary from Valgrind on 'menuconfig' (ARCH=x86) before the fix:

	LEAK SUMMARY:
	   definitely lost: 344,616 bytes in 14,355 blocks
	   ...

Summary after the fix:

	LEAK SUMMARY:
	   definitely lost: 344,568 bytes in 14,352 blocks
	   ...
Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/kconfig/zconf.y

@@ -236,8 +236,10 @@ symbol_option_list:
 	| symbol_option_list T_WORD symbol_option_arg
 {
 	const struct kconf_id *id = kconf_id_lookup($2, strlen($2));
-	if (id && id->flags & TF_OPTION)
+	if (id && id->flags & TF_OPTION) {
 		menu_add_option(id->token, $3);
+		free($3);
+	}
 	else
 		zconfprint("warning: ignoring unknown option %s", $2);
 	free($2);