Bluetooth: Add support for running SMP without a socket

When doing the pairing procedure we won't have an associated
socket, but we still have to do the SMP negotiation. This
adds support for encrypting the link and exchanging keys.

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