samples: rtu_server: add support for CDC ACM UART

Allow to use RTU server sample on any board that has supported
USB device controller. Although it is only a point to point
connection and does not represent a bus, it can, apart from
testing the server implementation, also be used practically
for example to control relays or to read ADC values via
USB connection without implementing custom USB class or driver.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>