examples/rpl-border-router: Fix two minor warnings.

 - Remove unused variable i in generate_routes.
 - Add cast to unsigned long for printf(..%lu..) in generate_routes.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
Merge pull request #797 from gebart/rpl-border-router-warnings

examples/rpl-border-router: Fix two minor warnings.