Support variant of `bootc install to-filesystem` omitting `bootupctl`, `fstab`

I'm playing with booting fedora-bootc as a live image. The end goal is a diskless setup where PXE downloads GRUB, GRUB downloads the initramfs and kernel, and dracut (`livenet` module) downloads the root squashfs (remotely built), mounts it as a live image, and lets ostree boot it. I don't need day-two updates. If my understanding is correct, this resembles how Fedora CoreOS runs from RAM.

I put together a working proof of concept (below) where I make a disk image with a GRUB boot partition, ESP, boot partition, and root partition, and run `bootc install to-filesystem` on the root partition. `bootc install to-filesystem` installs fedora-bootc to the root partition, writes `fstab`, and runs `bootupctl` to install GRUB to the boot partition. However, I end up deleting the generated `fstab` and disregarding the boot partition, since I really only need the squashfs of the root partition without `fstab`, and I'll roll my own GRUB config (writing my own `/boot/loader/entries/ostree-1.conf`).

In other words, this workflow would be much simpler if `bootc install to-filesystem` had an option to only install the container image to a given root, and omit writing `fstab` and running `bootupctl`. Are there any considerations I'm not thinking of that would make booting fedora-bootc as a live image a bad idea?

## Proof of concept

Constructing the squashfs:

```sh
#!/usr/bin/env bash

mkdir output

TMPDIR=$(mktemp --directory --tmpdir=output)
mkdir ${TMPDIR}/{efi,boot,root}

truncate ${TMPDIR}/disk.raw --size 16GiB
# boot partition needs to be on the same disk image, since otherwise bootupctl will complain
sfdisk ${TMPDIR}/disk.raw << EOF
label: gpt
label-id: $(uuidgen)
start=1M, size=1M, type="BIOS boot", uuid="$(uuidgen)"
size=100M, type=uefi, uuid="$(uuidgen)"
size=8G, type=linux, uuid="$(uuidgen)"
type=linux, uuid="$(uuidgen)"
EOF
DEVICE=$(sudo losetup --nooverlap --show --partscan --find ${TMPDIR}/disk.raw)

sudo mkfs.fat ${DEVICE}p2
sudo mount ${DEVICE}p2 ${TMPDIR}/efi

bootuuid=$(uuidgen)
sudo mkfs.ext4 -O "^orphan_file" -U ${bootuuid} ${DEVICE}p3
sudo mount ${DEVICE}p3 ${TMPDIR}/boot

rootuuid=$(uuidgen)
sudo mkfs.ext4 -O "^orphan_file" -U ${rootuuid} ${DEVICE}p4
sudo mount ${DEVICE}p4 ${TMPDIR}/root

sudo podman build --tag fedora-bootc-live - < /usr/lib/dracut/dracut.conf.d/01-livenet.conf
RUN mkdir /tmp/dracut ; dracut --rebuild \$(find /usr/lib/modules -name initramfs.img)
RUN echo "root" | passwd -s root
EOF

# without --skip-fetch-check: ERROR Installing to filesystem: Verifying fetch: Creating importer: failed to invoke method OpenImage: failed to invoke method OpenImage: pinging container registry localhost: Get "https://localhost/v2/": dial tcp [::1]:443: connect: connection refused
sudo podman run \
--rm \
--privileged \
--pull=newer \
--security-opt label=type:unconfined_t \
--volume ./${TMPDIR}/root:/var/root \
--volume ./${TMPDIR}/boot:/var/root/boot \
--volume ./${TMPDIR}/efi:/var/root/boot/efi \
--volume /var/lib/containers:/var/lib/containers \
--pid host \
localhost/fedora-bootc-live:latest \
bootc install to-filesystem --generic-image --skip-fetch-check /var/root

sudo mount -o remount,rw ${DEVICE}p4 ${TMPDIR}/root
OSTREE_DEPLOY=$(cd ${TMPDIR}/root ; find ostree/boot.1/default -name 0 -type l | head -1)
sudo rm ${TMPDIR}/root/${OSTREE_DEPLOY}/etc/fstab
KERNEL_DIR=$(find ${TMPDIR}/root/${OSTREE_DEPLOY}/usr/lib/modules -mindepth 1 -maxdepth 2 -type d | head -1)
INITRAMFS_IMG=output/initramfs.img
VMLINUZ=output/vmlinuz
sudo cp ${KERNEL_DIR}/initramfs.img ${INITRAMFS_IMG}
sudo cp ${KERNEL_DIR}/vmlinuz ${VMLINUZ}

sync
sudo umount ${TMPDIR}/boot
sudo umount ${TMPDIR}/efi
sudo umount ${TMPDIR}/root

sudo mkdir ${TMPDIR}/root/LiveOS
sudo dd if=${DEVICE}p4 of=${TMPDIR}/root/LiveOS/rootfs.img status=progress
SQUASHFS_IMG=output/squashfs.img
sudo mksquashfs ${TMPDIR}/root ${SQUASHFS_IMG}

sudo losetup --detach ${DEVICE}

sudo rm -rf ${TMPDIR}
```

Taking it for a spin. This makes a disk image with just a boot partition, installs GRUB manually, adds the squashfs / initramfs / kernel, and writes a config that references them:

```sh
#!/usr/bin/env bash

SQUASHFS_IMG=$1
INITRAMFS_IMG=$2
VMLINUZ=$3
OSTREE_DEPLOY=$4

mkdir output

TMPDIR=$(mktemp --directory --tmpdir=output)
mkdir ${TMPDIR}/{efi,boot}

truncate ${TMPDIR}/disk.raw --size 16GiB
sfdisk ${TMPDIR}/disk.raw << EOF
label: gpt
label-id: $(uuidgen)
start=1M, size=1M, type="BIOS boot", uuid="$(uuidgen)"
size=100M, type=uefi, uuid="$(uuidgen)"
type=linux, uuid="$(uuidgen)"
EOF
DEVICE=$(sudo losetup --nooverlap --show --partscan --find ${TMPDIR}/disk.raw)

sudo mkfs.fat ${DEVICE}p2
sudo mount ${DEVICE}p2 ${TMPDIR}/efi

bootuuid=$(uuidgen)
sudo mkfs.ext4 -O "^orphan_file" -U ${bootuuid} ${DEVICE}p3
sudo mount ${DEVICE}p3 ${TMPDIR}/boot

## --write-uuid is required. --with-static-configs doesn't work
sudo podman run \
--rm \
--privileged \
--pull=newer \
--security-opt label=type:unconfined_t \
--device ${DEVICE}:${DEVICE}:rwm \
--volume ./${TMPDIR}/boot:/var/root/boot \
--volume ./${TMPDIR}/efi:/var/root/boot/efi \
--pid host \
localhost/fedora-bootc-live:latest \
bootupctl backend install --device ${DEVICE} /var/root --write-uuid

sudo mkdir --parents ${TMPDIR}/boot/loader/entries
sudo tee ${TMPDIR}/boot/loader/entries/ostree-1.conf <

Beginne bei den Einstiegspunkten `bootc install to-filesystem` und `bootupctl backend install` und reproduziere dann den Proof of Concept aus dem Issue, um die aktuellen Schritte für die Einrichtung des Dateisystems und des Bootvorgangs zu verstehen. Als abgeschlossen gilt die Arbeit, wenn eine unterstützte Option das Image installieren kann, ohne `fstab` und `bootupctl` zu verwenden, und wenn der live-image-Workflow und seine Einschränkungen dokumentiert oder getestet sind.

Vom Indexierungsmodell aus dem Issue-Text verfasst.