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

Fix some insignificant warnings in the rpl-border-router example.

 - Remove unused variable i in generate_routes.
 - Add cast to unsigned long for printf(..%lu..) in generate_routes.

The patch also removes trailing whitespace in border-router.c

Clang gave the following warnings:

    border-router.c:250:23: warning: format specifies type 'unsigned long' but the argument has type 'uint32_t' (aka 'unsigned int') [-Wformat]
          ADD(") %lus\n", r->state.lifetime);
                 ~~~      ^~~~~~~~~~~~~~~~~
                 %u
    border-router.c:118:54: note: expanded from macro 'ADD'
        blen += snprintf(&buf[blen], sizeof(buf) - blen, __VA_ARGS__);      \
                                                         ^
    border-router.c:146:14: warning: unused variable 'i' [-Wunused-variable]
      static int i;
                 ^