octeon: fix image build

After b47f438 "build: remove image prefix from kernel files in KDIR", the
kernel image in $(KDIR) has a different name and could not be found by the
tar-file build step anymore, leading to the following error on the build
servers:

  cp: cannot stat `.../linux-octeon/lede-octeon-generic-kernel.bin': No such file or directory
  make[4]: *** [.../linux-octeon/tmp/lede-octeon-generic-ext4-sysupgrade.tar] Error 1

Adjust the path to the source kernel image in order to fix the problem.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

target/linux/octeon/image/Makefile

@@ -22,7 +22,7 @@ endef
 define Build/tar-file
 	mkdir -p $(KDIR)/sysupgrade-$(DEVICE_NAME)/
 	echo "BOARD=$(BOARD)" > $(KDIR)/sysupgrade-$(DEVICE_NAME)/CONTROL
-	$(CP) $(KDIR)/$(KERNEL_IMAGE) $(KDIR)/sysupgrade-$(DEVICE_NAME)/kernel
+	$(CP) $(KDIR_KERNEL_IMAGE) $(KDIR)/sysupgrade-$(DEVICE_NAME)/kernel
 	$(CP) $(KDIR)/root.$(1) $(KDIR)/sysupgrade-$(DEVICE_NAME)/root
 	(cd $(KDIR); $(TAR) cvf \
 		$@ sysupgrade-$(DEVICE_NAME))