6lowpan: move eui64 uncompress function

This function will be use in later functionality in other branches than
generic 6lowpan, so we move it to the global 6lowpan header.

Signed-off-by: Alexander Aring <aar@pengutronix.de>
Reviewed-by: Stefan <Schmidt&lt;stefan@osg.samsung.com>
Acked-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>