If we're using a Secure Channel Protocol, this will add overhead
in terms of the C-MAC appended to the C-APDU. This means in turn that
the useable length of the data field shrinks by a certain number of
bytes.
Let's make sure the SCP instances expose an 'overhead' property
of how much overhead they add - and that other commands use this to
determine the maximum command data field length.
Change-Id: I0a081a23efe20c77557600e62b52ba90a401058d
979c837286