flash/startup: extend "program" command to accept "exit"

This optional argument tells OpenOCD to exit after finishing (either
succesfully, or with an error) the programming sequence. Without it
OpenOCD stays running.

Change-Id: I6ecaf33ff985eea9a9cd02ff644a74403ae3e1e5
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2492


Tested-by: jenkins
Reviewed-by: Uwe Bonnes <bon@elektron.ikp.physik.tu-darmstadt.de>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>