modules: add 6LoWPAN over Bluetooth Low Energy

Adding support 6LoWPAN over Bluetooth Low Energy in kernel 3.18.
Creating new bluetooth_6lowpan group with CONFIG_6LOWPAN and CONFIG_BT_6LOWPAN.
Adding kernel object dependencies for 6LoWPAN over Bluetooth Low Energy.

Signed-off-by: Lukasz Duda <lukasz.duda@nordicsemi.no>
Signed-off-by: John Crispin <blogic@openwrt.org>

SVN-Revision: 44084