Automatically run `load` when setup mode starts

Gavin had trouble with configuration because it isn't loaded on startup.
I realized that I've been caught with the same mistake myself before, so
since at least 2 out of ~65 people have done this, it's clearly a real
usability problem.

Now the configuration is automatically loaded from flash when setup mode
starts, so there's no need to type `load` to edit the configuration the
Sink already has.  If you're changing all the settings, you'll have to
type them all anyway, so I don't think there will be any surprises
because of this change.

docs/console_config.md

 to change some settings while leaving others alone.  If there is no
 configuration, `No configuration` is printed instead.
 
+Starting with firmware version 1.3.0, this is automatically run when setup mode
+starts.
+
 #### write
 
 Usage: `write`

src/shell.c

     /* Get the current configuration */
     struct pdbs_config *cfg = pdbs_config_flash_read();
     if (cfg == NULL) {
-        chprintf(chp, "No configuration\r\n");
+        /* When we run this at shell startup, we want it to be quiet */
+        if (chp != NULL) {
+            chprintf(chp, "No configuration\r\n");
+        }
         return;
     }
 
     pdb_config = cfg;
     pdbs_dpm_data = cfg->dpm_data;
 
+    /* Load the configuration before the shell starts */
+    cmd_load(NULL, 0, NULL);
+
     while (true) {
         /* Print the prompt */
         chprintf(chp, "PDBS) ");

src/usbcfg.c

  * Serial Number string.
  */
 static const uint8_t vcom_string3[] = {
-    USB_DESC_BYTE(12),                    /* bLength.                         */
+    USB_DESC_BYTE(20),                    /* bLength.                         */
     USB_DESC_BYTE(USB_DESCRIPTOR_STRING), /* bDescriptorType.                 */
-    '1', 0, '.', 0, '2', 0, '.', 0, '0', 0
+    '1', 0, '.', 0, '3', 0, '.', 0, '0', 0, '-', 0, 'd', 0, 'e', 0, 'v', 0
 };
 
 /*