The .config file is created when running "make config", "make
menuconfig" or "make xconfig" (prior to actually building the kernel).
It needs to be there and correctly reflect how the kernel is actually
built for most out-of-kernel drivers to work.

Some kernels store their config in /proc/config.gz, if this is the
case for yours the following might be enough to get your modules
correctly built:

mkdir /usr/src/linux
zcat /proc/config.gz > /usr/src/linux/.config

-Brett.