scripts: west_commands: openocd: Use 'reset init' by default

Instead of 'reset halt', default the command to reset and halt the target
to 'reset init'.

OpenOCD strictly requires 'reset init' to halt and prepare the device
before flash programming.
See https://openocd.org/doc/html/Flash-Commands.html

 chapter
"12.2 Preparing a Target before Flash Programming."

For targets implementing the init hook, it will allows to perform some
target specific init improvements (such as speed flash improvements).
Targets not implementing this hook will end up executing the usual and
previous 'halt'.

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