octeon: fix image build

After b47f438d "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>