Bluetooth: Add simple SMP pairing negotiation

This implementation only exchanges SMP messages between the Host and the
Remote. No keys are being generated. TK and STK generation will be
provided in further patches.

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