Bluetooth: SMP: Add support for debug ECDH keys

This add following handling of LE SC debug keys:
- if debug keys are disabled locally generated public/private keys
  is  checked to not match debug keys by accident
- if debug keys are enabled those are used as local public/private
  keys pair allowing sniffing of encrypted air traffic
- tracking of remote device using debug keys
- debug keys are discarded after disconnection regardless of bonding
  flag being set or not
- due to HCI interface limitation debug keys are not supported for
  HCI based ECDH and compilation error will be rised for such
  configuration

Change-Id: Ie147b1893c4a6b9542b4d2c261973a7acbd8de96
Signed-off-by: Szymon Janc <ext.szymon.janc@tieto.com>