Add support for the firmware version extension

This commit adds support for the firmware version extension, as
described at http://www.fwupd.org/developers S. USB Firmware Version
Extensions.  This gives a new way of reading the firmware version, which
is apparently preferred by fwupd.  I'd kinda like to be able to use
fwupd to update the device's firmware starting with the new minor
version, so this is a first step towards that.

src/usbcfg.c

 };
 
 /* Configuration Descriptor tree for a CDC.*/
-static const uint8_t vcom_configuration_descriptor_data[67] = {
+static const uint8_t vcom_configuration_descriptor_data[76] = {
     /* Configuration Descriptor.*/
-    USB_DESC_CONFIGURATION(67,            /* wTotalLength.                    */
-            0x02,          /* bNumInterfaces.                  */
+    USB_DESC_CONFIGURATION(76,            /* wTotalLength.                    */
+            0x03,          /* bNumInterfaces.                  */
             0x01,          /* bConfigurationValue.             */
             0,             /* iConfiguration.                  */
             0xC0,          /* bmAttributes (self powered).     */
     USB_DESC_ENDPOINT     (USBD1_DATA_REQUEST_EP|0x80,    /* bEndpointAddress.*/
             0x02,          /* bmAttributes (Bulk).             */
             0x0040,        /* wMaxPacketSize.                  */
-            0x00)          /* bInterval.                       */
+            0x00),         /* bInterval.                       */
+    /* Interface Descriptor.*/
+    USB_DESC_INTERFACE    (0x02,          /* bInterfaceNumber.                */
+            0x00,          /* bAlternateSetting.               */
+            0x00,          /* bNumEndpoints.                   */
+            0xFF,          /* bInterfaceClass (Communications
+                              Interface Class, CDC section
+                              4.2).                            */
+            'F',           /* bInterfaceSubClass (Abstract
+                              Control Model, CDC section 4.3).   */
+            'W',           /* bInterfaceProtocol (AT commands,
+                              CDC section 4.4).                */
+            3),            /* iInterface.                      */
 };
 
 /*