package/nvidia-driver: fixup libraries according to README

package/nvidia-driver: fixup libraries according to README

Create a symlink for libglx.so and drop libnvidia-wfb.so (aka libwfb.so)
since all selectable xserver versions in Buildroot provide their own.

VDPAU libraries should be installed into /usr/lib/vdpau/

https://download.nvidia.com/XFree86/Linux-x86_64/390.67/README/installedcomponents.html

Also, allow specifying target subdirectory per library and respect it in
the install loop.

Signed-off-by: Vincent Fazio <vfazio@xes-inc.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>