west: runners: update the rtt implementation for openocd

Previously, rtt start would always fail because the target
(i.e. firmware) had not been started.

```
Info : rtt: Searching for control block 'SEGGER RTT'
Info : rtt: No control block found
```

When the command is 'rtt', run the binary via gdb before calling
'rtt start'. Firmware calls `SEGGER_RTT_Init()` shortly after
init which allows OpenOCD to find the RTT control block.

Similarly, only start the 'rtt server' after calling
'rtt start' to avoid any potential race conditions
internally within OpenOCD.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>