Bluetooth: Detect SMP SC debug keys

We need to be able to detect if the remote side used a debug key for the
pairing. This patch adds the debug key defines and sets a flag to
indicate that a debug key was used. The debug private key (debug_sk) is
also added in this patch but will only be used in a subsequent patch
when local debug key support is implemented.

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