User can now enable extension callbacks in RPDO/TPDO handling paths
simply by configuration flags instead of modifying sources or by passing
the defines directly to the compiler.
* Merge two parts of CO_PDO_receive() into single common code
* Add CO_RPDO_initCallbackPre() for received RPDO messages
This optional mechanism allows to immediatelly wake the thread that
processes RPDO, without waiting for its next tick.
* Add CO_SYNC_initCallbackPre() for received SYNC messages
This optional mechanism allows to immediatelly wake the thread that
processes SYNC, without waiting for its next tick.
