samples: usb: add usb.rst and README.rst for CDC ACM sample

Signed-off-by: Johann Fischer <j.fischer@phytec.de>

samples/subsys/usb/cdc_acm/README.rst

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+.. _usb_cdc-acm:
+
+USB CDC ACM Sample Application
+####################################
+
+Overview
+********
+
+This sample app demonstrates use of a USB Communication Device Class (CDC)
+Abstract Control Model (ACM) driver provided by the Zephyr project.
+Received data from the serial port is echoed back to the same port
+provided by this driver.
+This sample can be found under :file:`samples/subsys/usb/cdc_acm` in the
+Zephyr project tree.
+
+Requirements
+************
+
+This project requires an USB device driver,
+which is available for Intel Quark* SE SoC based boards like
+:ref:`arduino_101`.
+For the Arduino 101 board you need a USB to 3.3V TTL serial cable.
+Connect it to UART1 as described in
+:ref:`Connecting Serial Output section <arduino_101>`.
+
+Building and Running
+********************
+
+Arduino 101
+===========
+
+To see the console output of the app, open a serial port emulator and
+attach it to the USB to TTL Serial cable. Build and flash the project:
+
+.. code-block:: console
+
+   $ cd samples/subsys/usb/cdc_acm
+   $ make BOARD=arduino_101 flash
+
+See :ref:`Flashing the x86 Application Core section <arduino_101>` for details.
+
+Running
+=======
+
+Plug the board into a host device, for example, a PC running Linux.
+The board will be detected as shown by the Linux dmesg command:
+
+.. code-block:: console
+
+   usb 9-1: new full-speed USB device number 112 using uhci_hcd
+   usb 9-1: New USB device found, idVendor=8086, idProduct=f8a1
+   usb 9-1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
+   usb 9-1: Product: CDC-ACM
+   usb 9-1: Manufacturer: Intel
+   usb 9-1: SerialNumber: 00.01
+   cdc_acm 9-1:1.0: ttyACM1: USB ACM device
+
+The app prints on serial output (UART1), used for the console:
+
+.. code-block:: console
+
+   Wait for DTR
+
+Open a serial port emulator, for example minicom
+and attach it to detected CDC ACM device:
+
+.. code-block:: console
+
+   minicom --device /dev/ttyACM1
+
+The app should respond on serial output with:
+
+.. code-block:: console
+
+   DTR set, start test
+   Baudrate detected: 115200
+
+And on ttyACM device, provided by zephyr USB device stack:
+
+.. code-block:: console
+
+   Send characters to the UART device
+   Characters read:
+
+The characters entered in serial port emulator will be echoed back.
+
+Troubleshooting
+===============
+
+If the ModemManager runs on your operating system, it will try
+to access the CDC ACM device and maybe you can see several characters
+including "AT" on the terminal attached to the CDC ACM device.
+You can add or extend the udev rule for your Arduino 101 to inform
+ModemManager to skip the CDC ACM device.
+For this example, it would look like this:
+
+.. code-block:: none
+
+   ATTRS{idVendor}=="8086" ATTRS{idProduct}=="f8a1", ENV{ID_MM_DEVICE_IGNORE}="1"
+
+You can use
+`/lib/udev/rules.d/77-mm-usb-device-blacklist.rules` as reference.

samples/subsys/usb/usb.rst

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+.. _usb-samples:
+
+USB Samples
+###############
+
+.. toctree::
+   :maxdepth: 1
+   :glob:
+
+   **/*