kconfig: do not allow more than one symbol to have 'option modules'

Previously, it was possible to have more than one symbol with the
'option modules' attached to them, although only the last one would
in fact control tristates.

Since this does not make much sense, only allow at most one symbol to
control tristates.

Note: it is still possible to have more than one symbol that control
tristates, but indirectly:

    config MOD1
        bool "mod1"
        select MODULES
    config MOD2
        bool "mod2"
        select MODULES
    config MODULES
        bool
        option modules

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>