Update kelvin renode script to properly support debugging

To debug Renode with GDB, the Renode simulation should be started by
GDB automatically when the debug server is attached to GDB. Clean up the
renode script to follow the same flow we used in springbok to make sure
GDB can capture the process reliably.

Change-Id: I416565bcb8b02b8b137fe2a7ebc822d9821607ac