nixos support for nvidia optimus

currently, only support for fully disabling nvidia is provided, which
is helpful for saving power/heat.

In the future, this should be extended so we can choose:

- nvidia only  (choose between nouveau/nvidia driver)
- IGP only
- Hybrid (choose between nouveau/nvidia driver, use the "bumblebee" package/daemon)

svn path=/nixos/trunk/; revision=32085