science/py-tensorflow: Fix the option CPU_OPTS=ON, it wasn't passing options to bazel.

The BAZEL_COPT variable was always left blank, while docs recommend passing optimization options to bazel.

Also replace .include with option helpers.

Testing performed: Ran with CPU_OPTS=ON on a simple computation and observed that warnings about missing SSE4.2 optimizations are gone.
Approved by: portmgr (unbreak the option; change to the recommended format)