Bluetooth: Add flags and setting for Secure Connections support

The MGMT_SETTING_SECURE_CONN setting is used to track the support and
status for Secure Connections from the management interface. For HCI
based tracking HCI_SC_ENABLED flag is used.

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