net: add ieee802154_6lowpan namespace

This patch adds necessary ieee802154 6lowpan namespace to provide the
inet_frag information. This is a initial support for handling 6lowpan
fragmentation with the inet_frag api.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
6lowpan: rename 6lowpan.c to 6lowpan_rtln.c
6lowpan: handling 6lowpan fragmentation via inet_frag api

This patch drops the current way of 6lowpan fragmentation on receiving
side and replace it with a implementation which use the inet_frag api.
The old fragmentation handling has some race conditions and isn't
rfc4944 compatible. Also adding support to match fragments on
destination and source address which is missing in the current
implementation.

This patch add also a lookup function for uncompressed 6LoWPAN header
size. This is needed to estimate the real size of fragmented packet.