Set version number, document USB descriptors

The iSerial descriptor now holds the firmware version number.  This is
1.0.0 since I don't expect any major API changes any time soon.  The USB
descriptors have been documented in docs/console_config.md so there
will be no questions as to how to identify the devices and what their
"serial numbers" mean.

docs/console_config.md

 # PD Buddy Sink Serial Console Configuration Interface
 
+Version 1.0.0, 2017-06-05
+
 The PD Buddy Sink can be put into setup mode by holding the Setup button while
 plugging it into a computer.  In this mode, the device does not perform any USB
 Power Delivery communications, instead running a configuration console over a
 The `i` field holds the fixed current of the configuration object, in amperes.
 The field's value is a floating-point decimal number, followed by a space and a
 capital A.  For example: `2.25 A`
+
+## USB Descriptors
+
+The PD Buddy Sink can be identified by the following USB device descriptors:
+
+* idVendor: 0x1209 (InterBiometrics, or pid.codes)
+* idProduct: 0x9DB5 (PD Buddy Sink)
+
+The device's firmware version number is given in the iSerial descriptor.  The
+version number follows [Semantic Versioning][].  The serial console
+configuration interface is the API that the version number describes.
+
+[Semantic Versioning]: http://semver.org/

src/usbcfg.c

  * Serial Number string.
  */
 static const uint8_t vcom_string3[] = {
-    USB_DESC_BYTE(8),                     /* bLength.                         */
+    USB_DESC_BYTE(12),                    /* bLength.                         */
     USB_DESC_BYTE(USB_DESCRIPTOR_STRING), /* bDescriptorType.                 */
-    '0' + CH_KERNEL_MAJOR, 0,
-    '0' + CH_KERNEL_MINOR, 0,
-    '0' + CH_KERNEL_PATCH, 0
+    '1', 0, '.', 0, '0', 0, '.', 0, '0', 0
 };
 
 /*