net: ipv6: Bluetooth address fix

If IPv6 address is generated from Bluetooth MAC address,
then the Universal/Local bit must not be toggled or touched
at all. See RFC 7668 ch 3.2.2 for details.

Because this change is not compatible with older Linux kernel
BT IPSP support, the old behavior can be enabled by setting
CONFIG_NET_L2_BLUETOOTH_ZEP1656 option.

Change-Id: I05d48723b70f1eb60fbd46107ef6a2a4e8f9154a
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>