scripts: runners: new openocd fixes and features

Let the user specify these arguments as many times as they want:

--cmd-pre-init
--cmd-pre-load
--cmd-post-verify

This makes it a bit easier to handle scripts that need to do a few
things in a row depending on conditions.

Handle --cmd-pre-init and the port arguments properly in the debug
related targets.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>