Bluetooth: Start SMP procedure

Start SMP procedure for LE connections. This modification intercepts
l2cap received frames and call proper SMP functions to start the SMP
procedure. By now, no keys are being used.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@openbossa.org>
Signed-off-by: Anderson Briglia <anderson.briglia@openbossa.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>