snippets: cdc-acm-console: Redirect Shell target to USB

To easily enable Shell over USB. Tested with:
```
west build -b nrf52840dk/nrf52840 \
           -S cdc-acm-console` \
           samples/subsys/shell/shell_module
```

Signed-off-by: Luis Ubieda <luisf@croxel.com>