kconfig: introduce menu type enum

Currently, menu->prompt->type is checked to distinguish "comment"
(P_COMMENT) and "menu" (P_MENU) entries from regular "config" entries.
This is odd because P_COMMENT and P_MENU are not properties.

This commit introduces menu type enum to distinguish menu types more
naturally.

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