usbdev: Add board unique serial string support

iSerialNumber field in the device descriptor can be used to determining the
board when multiple boards connected to the same host. So add feature to change
serial string by board unique ID dynamically.
To use this feature, user must be implement the board_usbdev_serialstr() logic.

refs #13909

drivers/usbdev/composite.c

@@ -118,7 +118,9 @@ static const struct usbdevclass_driverops_s g_driverops =
 
 const char g_compvendorstr[]  = CONFIG_COMPOSITE_VENDORSTR;
 const char g_compproductstr[] = CONFIG_COMPOSITE_PRODUCTSTR;
+#ifndef CONFIG_COMPOSITE_BOARD_SERIALSTR
 const char g_compserialstr[]  = CONFIG_COMPOSITE_SERIALSTR;
+#endif
 
 /****************************************************************************
  * Private Functions