kconfig: link menus to a symbol

Currently, there is no direct link from (struct symbol) to (struct menu).

It is still possible to access associated menus through the P_SYMBOL
property, because property::menu is the relevant menu entry, but it
results in complex code, as seen in get_symbol_str().

Use a linked list for simpler traversal of relevant menus.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>

scripts/kconfig/expr.h

@@ -108,6 +108,9 @@ struct symbol {
 	 */
 	tristate visible;
 
+	/* config entries associated with this symbol */
+	struct list_head menus;
+
 	/* SYMBOL_* flags */
 	int flags;
 
@@ -222,6 +225,8 @@ struct menu {
 	 */
 	struct symbol *sym;
 
+	struct list_head link;	/* link to symbol::menus */
+
 	/*
 	 * The prompt associated with the node. This holds the prompt for a
 	 * symbol as well as the text for a menu or comment, along with the