Bug#815515: solved with preseed

[Bob]

Bug report was a bit premature but maybe this helps other users of 
live-build.

Found out nvidia-legacy-check can be disabled by preseeding it.
For a live-build environment, create the file 
config/preseed/nvidia.cfg.chroot and add:

nvidia-legacy-check nvidia-driver/check-for-unsupported-gpu boolean false