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. This pull fixes most of these calls
to use as DODAG ID a routable v6 address which belongs to the root.
This is a MUST in [RFC 6550, section 8.2](http://tools.ietf.org/html/rfc6550#section-8.1)

Two calls are not fixed by this pull:
* platform/avr-ravenusb/contiki-raven-main.c
* platform/minimal-net/contiki-main.c

This pull also makes a very minor code style adjustment