Several functions require traversing menu entries sequentially. This
commit introduces some helpers to simplify such operations.

The menu_next() function facilitates depth-first traversal:

 1. Descend to the child level if the current menu has one
 2. Move to the next sibling at the same level if available
 3. Ascend to the parent level if there is no more child or sibling

The menu_for_each_sub_entry() macro iterates over all submenu entries
using depth-first traverse.

The menu_for_each_entry() macro is the same, but over all menu entries.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>