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Sometimes Arduino-based USB devices don't work because some hardware
(like KVM switches) gets confused by the CDC sub-devices.
This change makes it relatively easy to disable CDC at compiletime.
Disabling it of course means that the serial console won't work anymore,
so you need to use the reset button when flashing.

CDC_DISABLED is also used in ArduinoCore-samd for the same purpose.

based on
https://github.com/gdsports/usb-metamorph/tree/master/USBSerPassThruLine

See also https://github.com/NicoHood/HID/issues/225 and
https://github.com/arduino/Arduino/issues/6387 and
https://forum.arduino.cc/index.php?topic=545288.msg3717028#msg3717028