kconfig: fix randconfig tristate detection

Because the modules' symbole (CONFIG_MODULES) may not yet be set when
we check a symbol's tristate capabilty, we'll always find that tristate
symbols are booleans, even if we randomly decided that to enable modules:
sym_get_type(sym) always return boolean for tristates when modules_sym
has not been previously set to 'y' *and* its value calculated *and* its
visibility calculated, both of which only occur after we randomly assign
values to symbols.

Fix that by looking at the raw type of symbols. Tristate set to 'm' will
be promoted to 'y' when their values will be later calculated.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>