cyg_modbus_response() call this function
is used to provide the MODBUS server with a valid response to a
request handed to the backend for processing. This call assumes that
the original PDU buffer (as referenced through the
ctx handle) contains the response data.
This “PDU buffer” re-use can simplify the backend processing, since it no longer needs to dynamically manage buffers that are handed-off to the main MODBUS server control.