samples: usb: add shell sample for new USB support

The sample enables new experimental USB device support
and the shell function.

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

samples/subsys/usb/shell/CMakeLists.txt

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+# Copyright (c) 2022 Nordic Semiconductor ASA
+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.20.0)
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(usb-shell)
+
+FILE(GLOB app_sources src/*.c)
+target_sources(app PRIVATE ${app_sources})

samples/subsys/usb/shell/README.rst

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+.. _usb_shell-app:
+
+USB support shell sample
+########################
+
+Overview
+********
+
+The sample enables new experimental USB device support and the shell function.
+It is primarily intended to aid in the development and testing of USB constoller
+drivers and new USB support.
+
+Building and flashing
+*********************
+
+Assuming the board has a supported USB device controller, the example can be
+built like:
+
+.. zephyr-app-commands::
+   :zephyr-app: samples/subsys/usb/shell
+   :board: reel_board
+   :goals: flash
+   :compact:
+
+Sample shell interaction
+========================
+
+.. code-block:: console
+
+   uart:~$ usbd defaults
+   dev: USB descriptors initialized
+   uart:~$ usbd config add 1
+   uart:~$ usbd class add foobaz 1
+   dev: added USB class foobaz to configuration 1
+   uart:~$ usbd init
+   dev: USB initialized
+   uart:~$ usbd enable
+   dev: USB enabled

samples/subsys/usb/shell/prj.conf

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+CONFIG_SHELL=y
+CONFIG_USB_DEVICE_STACK_NEXT=y
+CONFIG_USBD_SHELL=y
+CONFIG_USBD_LOOPBACK_CLASS=y
+
+CONFIG_LOG=y
+CONFIG_USBD_LOG_LEVEL_WRN=y
+CONFIG_UDC_DRIVER_LOG_LEVEL_WRN=y

samples/subsys/usb/shell/sample.yaml

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+sample:
+  name: USB support shell sample
+tests:
+  sample.usbd.shell:
+    platform_allow: nrf52840dk_nrf52840 frdm_k64f
+    depends_on: usb_device
+    tags: usb

samples/subsys/usb/shell/src/main.c

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+/*
+ * Copyright (c) 2022 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr/kernel.h>
+
+void main(void)
+{
+}