how to build just only a kernel module from linux source tree:

1. steal Module.symvers from currently running kernel. It's usually in linux-headers packages. Copy it to the source tree root.
2. make oldconfig prepare modules_prepare
3. make M=path/to/subdir

If it errors out with missing symbols, compile modules that have them. Then do:
4. grep <missing symbol> path/to/dependency/Module.symvers >> Module.symvers

and rebuild the module you want

After that drop all built *.ko's to /lib/modules and run depmod, so it figures out dependencies for modprobe.