debug: Move away from openocd and make it generic

The exported structures that were originally introduced for OpenOCD have
since then been reused for other debugger plugins, including PyOCD and
Segger J-Link.

Rename the Kconfig option and the implementation from openocd to debug
thread info, so that it reflects the fact that this is no longer
specifically tied to OpenOCD.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>