boards: bl654_usb: use CDC ACM UART as serial backend

This patch adds new board specific Kconfig option
BL654_USB_SERIAL_BACKEND_CDCACM, which is enabled by
default except USB_DEVICE_BLUETOOTH class is used.
Applications that depend on usb_enable() or do not
require composite configuration cannot be used
with BL654 USB board without user intervention.

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