Use a more standard kernel binary location.

[adam900710]

And using `vmlinuz` would make grub-config work better.

The problem I can see is the migration from the initial `Image` to the new `vmlinuz`.

For Uboot based extlinux config, currently we just don't touch `/boot/extlinux/extlinux.conf` at all if it already exists.
Thus if we just do a rename, then it can cause boot failures for users with uboot/systemd-boot.