x86: onie-installer: wire up sysupgrade via ONIE install mode

x86: onie-installer: wire up sysupgrade via ONIE install mode

This adds support for sysupgrade on ONIE-installed systems.
The install is chained through ONIE (using the ONIE installer image),
rather than attempting to manually upgrade the partition.
The idea is to allow future OpenWRT installs flexibility to use
a different partition table. By putting the installer in charge
of setting up the file system partition, the upgrade process needs
to have no knowledge of the internals of the image.

Config preservation is accomplished by appending the sysupgrade .tar.gz
to the ONIE installer image. Of course this also works for a clean
install using a sysupgrade.tar.gz created via `sysupgrade -b`.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Signed-off-by: Keno Fischer <keno@juliahub.com>
Link: https://github.com/openwrt/openwrt/pull/23062
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>