examples/rpl-border-router: Fix two minor warnings.
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 <d097f2c6c8fee73e3ea75e1d62412e3886346faa@eistec.se>

bluerover/6lbr,MohamedSeliem/contiki,bluerover/6lbr,arurke/contiki,arurke/contiki,bluerover/6lbr,MohamedSeliem/contiki,bluerover/6lbr,MohamedSeliem/contiki,arurke/contiki,bluerover/6lbr,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,MohamedSeliem/contiki,bluerover/6lbr,arurke/contiki,MohamedSeliem/contiki,bluerover/6lbr,arurke/contiki,arurke/contiki