kconfig: Create links to main menu items in search

When one searches for a main menu item, links aren't created for it like
with the rest of the symbols.

This happens because we trace the item until we get to the rootmenu, but
we don't include it in the path of the item. The rationale was probably
that we don't want to show the main menu in the path of all items,
because it is redundant.

However, when an item has only the rootmenu in its path it should be
included, because this way the user can jump to its location.

Add a 'Main menu' entry in the 'Location:' section for the kconfig
items.

This makes the 'if (i > 0)' superfluous because each item with prompt
will have at least one menu in its path.

Signed-off-by: Ariel Marcovitch <arielmarcovitch@gmail.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>