mkosi: rework how we pass roothash/usrhash/other metadata to kernel-install
mkosi: rework how we pass roothash/usrhash/other metadata to kernel-install

Previously, we'd replace the main kernel-install script with one
generated by mkosi and pass the roothash/usrhash as final argument on
the command line. That's less than ideal though, since it collides with
the documented way kernel-install dropins are called (those arguments
are initrds, not root hashes according to the docs). Moreover, it leaves
no nice way to pass usrhash vs. roothash or pass other information.

Let's fix this all, by changing the script to look for the roothash in
$ROOTHASH and the usrhash in $USRHASH. Then pass these env vars when
invoking the script, and drop any additional arguments.

Also, add the $IMAGE_ID and $IMAGE_VERSION in a similar fashion and then
use those preferably to name the generated kernel images.

With all this in place we are fully "standards-compliant" again,
matching the documented kernel-install drop-in API, the kernel images
are named a lot more nicely, and the usrhash logic starts working too.

systemd/mkosi,systemd/mkosi