runners: openocd: add support for custom reset halt command

Add support for specifying a custom OpenOCD command to be used for
resetting and halting a target.

As noted in the OpenOCD documentation, not all targets support the
"reset halt" command for halt-on-reset.

Some targets support a software emulation via the "soft_reset_halt"
command. Other targets may require a custom command (e.g. a command
defined in the target configuration file).

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>