6lowpan: Moving generic compression code into 6lowpan_iphc.c

Because the IEEE 802154 and Bluetooth share the IP header compression
and uncompression code, the common code is moved to 6lowpan_iphc.c
file.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Acked-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

diff --cc net/ieee802154/6lowpan.c
Simple merge

diff --cc net/ieee802154/6lowpan.h
Simple merge

diff --cc net/ieee802154/6lowpan_iphc.c
Simple merge

diff --cc net/ieee802154/Makefile
Simple merge