When `stdio_cdc_acm` is used, prefer selecting the USB devices with
vendor `RIOT-os.org` and model `$(BOARD)` and only fall back
to `$(TTY_BOARD_FILTER)` if no such TTY was found.
This makes it possible to add a TTY board filter matching only an
integrated USB to UART bridge, but still do the right thing when
the user explicitly uses `stdio_cdc_acm` instead of the default
`stdio_uart`.
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