6lowpan: add nhc module for GHC routing extension header detection

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