kconfig: implement the `mainmenu' directive

If specified, the directive must be placed at the top of the Kconfig file.

We need to change the grammar to make the mainmenu directive set the
`rootmenu' prompt. This reflect how menu_add_prompt() works internally, ie.
set the prompt of the `current_entry', pointing originally to `rootmenu'.

Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Michal Marek <mmarek@suse.cz>