RPL: update to spec, add autoconf

This commit touches most of the RPL stack. It makes two main changes:

1. Update RPL data structures to be spec compliant. There were still a
couple of lingering data structures that didn't match the spec that are
now updated. Also, some of the processing and assumptions about the
content of received messages has been updated to better match the spec.

2. Add the prefix information option. This option extends DIO messages
to allow nodes to perform autoconfiguration with just RPL messages.

This commit adds a new RPL `#define` configuration option:

1. `RPL_ADDR_AUTOCONF`: if `1`, the node will set its address based on a
prefix received from a DIO message.

These changes have been briefly tested to be compatible with the Contiki
implementation of RPL.
BLIP: removed printf and whitespace from 6lowpan
BLIP+RPL: updating /apps

This commit updates the Makefile and cleans up the apps relevant to BLIP
and RPL.

Added: rpl.extra to make it easier to compile apps with RPL.