/**

    @page shellmatta_ymodem Shellmatta ymodem

    To be able to pass binary files to the shellmatta there is a ymodem module
    implemented.
    The ymodem reception can be started from a shellmatta command and handles
    the handshaking as well as the data transmission.

    As the shellmatta does not use dynamic memory every packet is passed
    directly to the application using a callback.
    Only the currently received packet is stored in the shellmatta internally.

    In order to transmit the start NCKs to the sender you need to poll the
    shellmatta_processData() function cyclically even if no new data has been
    received.
    Do this in 0.1s - 1s periods (depending on the latency you are after).


    The shellmatta_ymodem_start() function will return and the command it was
    called from will not be executed again. The rest of the ymodem processing
    will be handled by the shellmatta. The data will be passed using the
    callbacks registered during the start.

    To cancel the ymodem (e.g. timeout) just call shellmatta_ymodem_cancel()


    @section shellmatta_ymodem_sequence Basic sequence

    @startuml
        Application -> Shellmatta: shellmatta_ymodem_start()
        Shellmatta -> Host: send NCK to start transmission
        Shellmatta -> Application: return
        loop until ymodem transmission starts (or is canceled)
            Application -> Shellmatta: shellmatta_processData(null)
            Shellmatta -> Host: send NCK to start transmission
        end
        loop until shellmatta transmission is complete
            Host -> Shellmatta: Data transmission
            Shellmatta -> Application: packetRcv() callback
        end
        Shellmatta -> Application: transmissionComplete() callback
    @enduml

    While the ymodem is active no command can be executed.
    When the ymodem is canceled from the Host the data received afterwards will
    be processed as usual.

*/