target: add async algorithm timeout

target: add async algorithm timeout

An issue was observed when using an async algorithm with a target that had
not been previously reset beforehand. The target would enter a infinite
loop within target_run_flash_async_algorithm.

Add a timeout that will at least prevent this issue from happening. and also
suggest the user resets the target.

Change-Id: I5277e0d64e252d3d353e8d5bc9889a37fdc63060
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/949
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>