Bluetooth: Add MITM mechanism to LE-SMP

To achive Man-In-The-Middle (MITM) level security with Low Energy,
we have to enable User Passkey Comparison.  This commit modifies the
hard-coded JUST-WORKS pairing mechanism to support query via the MGMT
interface of Passkey comparison and User Confirmation.

Signed-off-by: Brian Gix <bgix@codeaurora.org>
Acked-by: Marcel <Holtmann&lt;marcel@holtmann.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>