boards: add common configuration for CDC ACM UART

Many boards have similar code to configure the USB and CDC ACM UART that
they want to use as a logging or shell backend. Some of them have an
incorrect or incomplete configuration.

These boards do not have a built-in debug adapter, but a SoC with a USB
device controller and a bootloader with USB device support. Introduce
common CDC ACM UART configuration that these boards should use for
logging or shell backend to avoid duplicate or incorrect configuration.

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