fff-sysupgrade: Security question with yes/no

Fixes: #31

Changes from Bjoerns ticket. Thanks.

CC: freifunk@bjoernengel.de
Signed-off-by: Tim Niemeyer <tim@tn-x.org>
Reviewed-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Tobias Klaus <tk+ff@meskal.net>