kconfig: don't emit warning upon rootmenu's prompt redefinition

This silences the warning printed upon prompt redefinition for the rootmenu.
We will encounter this redefinition when a "mainmenu" statement is specified and
override the default prompt.

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