west: runners: openocd: Add option to start RTT server during debug

Add `--rtt-server` option to start RTT server when the OpenOCD
launched from `debug`, `debugserver`, and `attach` commands.

This option starts the RTT server and opens a port,
but RTT logging must be done externally.
That is, you must connect to port 5555 using `telnet` or `nc`
in another terminal to view the RTT log.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>