west: runners: Implement the west rtt command for openocd

This was non-trivial, as openocd is a bit weird to work with. Using only
commands passed with '-c' arguments, I couldn't get it to reliably resume
(or just not halt) the target when started. I tried using the 'sleep'
command, and various 'configure -event XX { resume }' events, but nothing
panned out, as it seems to always halt after all `-c` commands have been
run.

To avoid that, this waits for the TCL RPC port to be up, and sends a
resume command there. This works reliably.

Signed-off-by: Tobias Pisani <mail@topisani.dev>