When shutting down, send SIGTERM, not SIGINT

When shutting down, send SIGTERM, not SIGINT

SIGINT should be from the user, not a script.  It also doesn't work as
reliably to shut down processes, as it's not always interpreted as a
termination request.  In addition, it causes KeyboardInterrupt exceptions in
the worker processes, which can interfere with our exception handling.

(Bitbake rev: e5f6e0e9de4c6d1dfdd269d2bf7f83c00c415a27)

Signed-off-by: Chris Larson <clarson@kergoth.com>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>