IEEE802.15.4: 6LoWPAN basic support

This patch provides base support for transmission of IPv6 packets as
well as the formation of IPv6 link-local addresses and statelessly
autoconfigured addresses on top of IEEE 802.15.4 networks.

For more information please look at the RFC4944 "Compression Format
for IPv6 Datagrams in Low Power and Losst Networks (6LoWPAN).

Signed-off-by: Alexander Smirnov <alex.bluesman.smirnov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>