6lowpan: IPv6 link local address

According to the RFC4944 (Transmission of IPv6 Packets over
IEEE 802.15.4 Networks), chapter 7:

The IPv6 link-local address [RFC4291] for an IEEE 802.15.4 interface
is formed by appending the Interface Identifier, as defined above, to
the prefix FE80::/64.

  10 bits            54 bits                  64 bits
+----------+-----------------------+----------------------------+
|1111111010|         (zeros)       |    Interface Identifier    |
+----------+-----------------------+----------------------------+

This patch adds IPv6 address generation support for the 6lowpan
interfaces.

Signed-off-by: Alexander Smirnov <alex.bluesman.smirnov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>