Bluetooth: Introduce SMP_DBG macro for low-level debuging

The various inputs & outputs of the crypto functions as well as the
values of the ECDH keys can be considered security sensitive. They
should therefore not end up in dmesg by mistake. This patch introduces a
new SMP_DBG macro which requires explicit compilation with -DDEBUG to be
enabled. All crypto related data logs now use this macro instead of
BT_DBG.

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