Bluetooth: Add support for LE SC numeric comparison

After the Pairing Confirm and Random PDUs have been exchanged in LE SC
it's time to generate a numeric comparison value using a new smp_g2
cryptographic function (which also builds on AES-CMAC). This patch adds
the smp_g2 implementation and updates the Pairing Random PDU handler to
proceed with the value genration and user confirmation.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>