efi_loader: correct shortening of device-paths

We use short device-paths in boot options so that a file on a block device
can be found independent of the port into which the device is plugged.

Usb() device-path nodes only contain port and interface information and
therefore cannot identify a block device.
UsbWwi() device-path nodes contain the serial number of USB devices.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/include/efi_api.h b/include/efi_api.h
index c57868a..404e9a1 100644
--- a/include/efi_api.h
+++ b/include/efi_api.h
@@ -610,6 +610,7 @@
 #  define DEVICE_PATH_SUB_TYPE_MSG_MAC_ADDR	0x0b
 #  define DEVICE_PATH_SUB_TYPE_MSG_UART		0x0e
 #  define DEVICE_PATH_SUB_TYPE_MSG_USB_CLASS	0x0f
+#  define DEVICE_PATH_SUB_TYPE_MSG_USB_WWI	0x10
 #  define DEVICE_PATH_SUB_TYPE_MSG_SATA		0x12
 #  define DEVICE_PATH_SUB_TYPE_MSG_NVME		0x17
 #  define DEVICE_PATH_SUB_TYPE_MSG_URI		0x18