Configure composite device

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With coreos it was possible to define a different disk/partition for the mutable part:
https://coreos.com/os/docs/latest/root-filesystem-placement.html

use-case: a bare metal installation

• installation via pxe coreos.inst.install_dev=sda
• have a immutable (small) disk on /dev/sda
• mutable data /var should go to /dev/sdb

with fedora-coreos i can configure/mount /var with following ignition:

---
variant: fcos
version: 1.0.0

storage:
  disks:
    - device: /dev/sdb
      wipe_table: true
      partitions:
        - label: var
          type_guid: 4F68BCE3-E8CD-4DB1-96E7-FBCAF984B709
  filesystems:
    - path: "/var"
      device: "/dev/sdb1"
      format: "ext4"
      wipe_filesystem: true
      label: "var"
systemd:
  units:
    - name: var.mount
      enabled: true
      contents: |
        [Mount]
        What=/dev/disk/by-partlabel/var
        Where=/var
        Type=ext4
        Before=ostree-remount.service
        [Install]
        WantedBy=local-fs.target

it works but: Im not sure the ostree-remount stuff will skip or is not able to mount correctly /var i i will hit various side-effects later on.

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may be duplicate of #94, but with another viewpoint

[ajeddeloh]

What you're describing (separate /var) is already supported. You shouldn't need Before=ostree-remount.service, in fact if anything you want it after (but Before=local-fs.target) to shadow that unit's /var mount.

Moving root isn't supported yet, but we do hope to support it eventually (that's what #94 is talking about).

Does that clarify things?