CoreSightTarget selected_core changes.
CoreSightTarget selected_core changes.

- Selected core is inited to None, then is set to the core number of the
  first created core.
- selected_core property returns None if no cores have been created.
- reset() performs a hardware reset if selected_core returns None.

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