Run user script from session ctor.
Run user script from session ctor.

- The user script is run before the Board instance is created, to give
  it a chance to influence early configuration.
- In order to still have handy objects in the user script's namespace,
  the namespace is updated in Session.open() to add those objects that
  aren't available when the script is first run.
- Add some symbols to user script namespace.

mbedmicro/pyOCD,mbedmicro/pyOCD,pyocd/pyOCD,pyocd/pyOCD,flit/pyOCD,mesheven/pyOCD,mesheven/pyOCD,mbedmicro/pyOCD,flit/pyOCD,mesheven/pyOCD