Bluetooth: L2CAP: Add configuration response handler

Adds L2CAP configuration response protocol definitions to be able handle
response from remote in reaction to issued local request to exchange
MTU. Current optimistic path of configuration initiated from local
assumes to set MTU contract in one iteration.

Change-Id: If86d96b37e9cd888ad10a624a79c864a137e027d
Signed-off-by: Arkadiusz Lichwa <arkadiusz.lichwa@tieto.com>