helper: shutdown command should return with 0 exit status

helper: shutdown command should return with 0 exit status

Commit a35712a85c42 caused a regression where command

openocd -c "echo a1; shutdown; echo a2"

always returned non-zero exit status to operating system,
even when commands before shutdown all succeeded. This patch
attempt to fix this.

Change-Id: I3f478c2c51d100af810ea0171d2fd4c8fcc657f3
Signed-off-by: Juha Niskanen <juha.niskanen@haltian.com>
Reviewed-on: http://openocd.zylin.com/2589
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>