kconfig: fix missing '# end of' for empty menu

Currently, "# end of ..." is inserted when the menu goes back to its
parent.

Hence, an empty menu:

  menu "Foo"
  endmenu

... ends up with unbalanced menu comments, like this:

  #
  # Foo
  #

Let's close the menu comments properly:

  #
  # Foo
  #
  # end of Foo

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>