1) Introduce USE_GPU_TARGET (inside docker) to determine if GPU capable
2) Use of USE_GPU_HOST now only in cyber|dev_start.sh

For build,

1) Mixed used of '--config=cpu' and '--config=gpu' not allowed
2) Make CPU build (i.e., '--config=cpu') on GPU capable machine possible
3) Error and exit attempting to run GPU build (`--config=gpu`) on CPU-only machines
4) Default to USE_GPU_TARGET if none of cpu|gpu build specified