Bluetooth: smp: Warn on enabling several Bluetooth options

Add warning about enabling the options below so that users are
aware that this is a security risk.

- CONFIG_BT_DEBUG_SMP
- CONFIG_BT_DEBUG_KEYS
- CONFIG_BT_OOB_DATA_FIXED
- CONFIG_BT_USE_DEBUG_KEYS
- CONFIG_BT_STORE_DEBUG_KEYS

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>