Bluetooth: make bluetooth 6lowpan as an option

Currently you can have bluetooth 6lowpan without ipv6 enabled. This
doesn't make any sense. With this patch you can disable/enable bluetooth
6lowpan support at compile time.

The current bluetooth 6lowpan implementation doesn't check the return
value of 6lowpan function. Nevertheless I added -EOPNOTSUPP as return value
if 6lowpan bluetooth is disabled.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>