target: hla correctly use target events

Because we were always running using target state TARGET_RUNNING target
algorithm's were a bit verbose compared to other targets.

This brings the hla target inline with the other targets.

Change-Id: I3a257fdc878b87660fac8b5eca22b421eee5b349
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1134
Tested-by: jenkins