If the kernel name is not KERNEL the update process fails on Generic x86_64 version 8.2.5, when the update file is a .img.gz. With .tar files as update all is fine.
The problem is in packages/sysutils/busybox/scripts/init. The update process says "kern-82 or SYSTEM is missing". For me it looks like the UPDATE_KERNEL variable is wrong and the patch init_patch.txt fixed it for me, but i dont know if there is any impact on other architectures than x86_64. I commented out 5 line, because i could not figure out what theyshould do and they change UPDATE_KERNEL and that seems to be the problem.
kernel command line
kodi7:~ # cat /proc/cmdline
root=/dev/ram0 rdinit=/init usbcore.autosuspend=-1 ip=dhcp boot=/dev/sda1 SYSTEM_IMAGE=sys-82 disk=/dev/sda5 ssh quiet BOOT_IMAGE=/kern-82
Hardware:
Asrock B360m itx with 8GB, 300GB, i3-8100 und uefi only mode
NUC D34010WYK with 8GB, 250GB