scripts: west_commands: runners: nrfutil: provide live feedback

nrfutil runner uses the batch-mode, so no live feedback is provided to
the user. However, batch-mode reports batch progress containing
human-readable strings of the operation being done. This patch changes
the _exec() implementation to parse the subprocess output in real-time,
logging to info the 'batch_update' reports. Note that only the first
batch update of a sequence (percentage = 0) is logged because first,
percentage resolution seems to be pretty bad, and, because logging
messages cannot be easily _appended_.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>