Update version number to 1.2.0

It's finally here!

docs/console_config.md

@@ -1,6 +1,6 @@
 # PD Buddy Sink Serial Console Configuration Interface
 
-Version 1.2.0-dev.1, 2018-01-13
+Version 1.2.0, 2018-01-13
 
 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 runs a configuration

src/usbcfg.c

@@ -199,10 +199,9 @@ static const uint8_t vcom_string2[] = {
  * Serial Number string.
  */
 static const uint8_t vcom_string3[] = {
-    USB_DESC_BYTE(24),                    /* bLength.                         */
+    USB_DESC_BYTE(12),                    /* bLength.                         */
     USB_DESC_BYTE(USB_DESCRIPTOR_STRING), /* bDescriptorType.                 */
-    '1', 0, '.', 0, '2', 0, '.', 0, '0', 0, '-', 0, 'd', 0, 'e', 0, 'v', 0,
-    '.', 0, '1', 0
+    '1', 0, '.', 0, '2', 0, '.', 0, '0', 0
 };
 
 /*