package/nvidia-driver: build the kernel module

Add option to build the nvidia.ko module. If CUDA is enabled on x86_64,
also build the nvidia-uvm.ko kernel module (for Unified Memory access),
which is required by the CUDA user-land library.

Substancially inspired by the corresponding Gentoo ebuild:
    http://sources.gentoo.org/cgi-bin/viewvc.cgi/gentoo-x86/x11-drivers/nvidia-drivers/nvidia-drivers-340.32.ebuild?revision=1.2&view=markup



[Thomas:
  - add quotes when using $(TARGET_CC) and other variables, since they
    can have spaces in their values
  - remove space after opening parenthesis and before closing parenthesis.]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>