Since commit 18895514dc5e ("kconfig: update to v3.18-rc6"), tristate
symbols cannot become 'm'.

In old days of Linux, CONFIG_MODULES was the special option hard-coded
in Kconfig.

The current Kconfig does not know which option is it. You need to
specify 'option modules' to teach Kconfig that it is the module
support switch.

Add 'option modules' to revive the module builds.

Fixes: 18895514dc5e ("kconfig: update to v3.18-rc6")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>