If you have several OS:es installed, don't use the combined image.
Put the kernel file in /boot of the other Linux, dd the ext4 openwrt root FS to a new partition, and do a resize2fs of it afterwards.
Modify the grub conf, and add the new entry for openwrt.
You're doing Sysupgrade help for x86_64 - #14 by frollic, but your primary os isn't openwrt.