kconfig: use linked list in get_symbol_str() to iterate over menus

Currently, get_symbol_str() uses a tricky approach to traverse the
associated menus.

With relevant menus now linked to the symbol using a linked list,
use list_for_each_entry() for iterating on the menus.

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