ieee802154: 6lowpan: rename to core

This patch renames the 6lowpan_rtnl.c file to core.c. 6lowpan_rtnl.c
contains functionality to put all 802.15.4 6LoWPAN functionality
together.
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/ieee802154/6lowpan/Makefile

 obj-y += ieee802154_6lowpan.o
 
-ieee802154_6lowpan-y := 6lowpan_rtnl.o rx.o reassembly.o tx.o
+ieee802154_6lowpan-y := core.o rx.o reassembly.o tx.o