Add meta-data to generic benchmark

[ddavis-2015]

@tensorflow/micro

Add meta-data strings to the binary. Meta-data is output each time binary is executed. See the README for a sample of the output.

If a Git repo. is not available at ${TENSORFLOW_ROOT}, an appropriate message is generated in the meta-data.

Changes to how the CMSISNN and XTENSA NN library download scripts work.

Adds run of generic benchmark with embedded model for default build of x86, Corstone-300, Xtensa to test scripts executed during CI.

bug=fixes #2495