Fix code style in rpl_set_root prototype
Make DODAG ID selection RFC 6550-compliant

Some calls to `rpl_set_root` select a hardcoded DODAG ID
(0x1111, 0x1100, 0, 0, 0, 0, 0, 0x0011)

This is against what RFC 6550 says. We change these calls
to select a DODAG ID corresponding to a routable v6 address
corresponding to the root
Merge pull request #718 from g-oikonomou/rpl-dodagid-compliance

Make DODAG ID selection RFC 6550-compliant