Add sthal_cli

This is a simple console app that overrides the default STHAL with a
mock one, useful for manual testing.

This is not production code and it is not designed to be rigorous and/
or complete, but rather as a quick tool that can be easily changed to
need.

Test: Manual running and verification of basic functionality
Change-Id: Ibd13a9dd83c163e02e76ce93f28d036d843854d5