sdk: provide explicit CONFIG_MODULES kconfig symbol

Extend the SDK Config.in file to explicitely declare a MODULES symbol with
the "option modules" flag set in order to prevent a kconfig segmentation fault
whenever a "depends m" dependency is encountered.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46514 3c298f89-4303-0410-b956-a3cf2f4a3e73