scripts: west: openocd runner: Add an option to specify target handle

Provide a way to specify the openocd target script handle used to
describe the core target.
In most target scripts '_TARGETNAME' is used, but it can happen that
'_TARGETNAME.foo' or '_CHIPNAME.bar' is used, specially on SoCs subject
to multicore.

Today, this option is required to enable rtos thread awareness,
using '$_TARGETNAME configure -rtos Zephyr' command, which is generated
by this runner.
It could be useful for similar needs in future.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>