net: Update 802.15.4 stack with net_mbuf and split sicslowpan

Move packetbuf from net_buf to net_mbuf, update 802.15.4 stack
with net_mbuf (which is smaller in size, and mostly wrapper for
packetbuf) and split sicslowpan(ipv6/sicslowpan.c|h) functionality
to compression and fragmentataion. This patch contains
null_compression and null_fragmentation which is exactly what
uip_driver.c|h does.

Change-Id: Ifee359e20aa57a50fb00c9ce049d9476b2ac67e3
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>