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>

snippets/cdc-acm-console/cdc-acm-console.overlay

@@ -7,6 +7,7 @@
 / {
 	chosen {
 		zephyr,console = &snippet_cdc_acm_console_uart;
+		zephyr,shell-uart = &snippet_cdc_acm_console_uart;
 	};
 };