tmpfiles: Ensure that /etc/coreos to flatcar symlink exists

[pothos]

The entries from the update or locksmith directive of a Container Linux
Config currently use the /etc/coreos/ folder for Ignition files:
printf "locksmith:\n reboot_strategy: off" | ct
results in:
{"ignition":{"config":{},"security":{"tls":{}},"timeouts":{},
"version":"2.3.0"},"networkd":{},"passwd":{},"storage":
{"files":[{"filesystem":"root", "path":"/etc/coreos/update.conf",
"contents":{"source":"data:,%0AREBOOT_STRATEGY%3D%22off%22",
"verification":{}},"mode":420}]},"systemd":{}}
With the pre-built rootfs this works because /etc/coreos is a symlink
to /etc/flatcar. However, with a fresh rootfs created by Ignition this
results in a /etc/coreos folder created by Ignition because there is no
symlink and the resulting files are ignored since only /etc/flatcar/ is
considered by update-engine and locksmith.
Set up the symlink and the target folder through a tmpfile directive to
ensure that it is there when Ignition runs even when with a new rootfs.

Fixes flatcar/Flatcar#190

How to use

Goes together with flatcar/init#28

Using an Ignition config like

{
  "ignition": {
    "version": "2.2.0"
  },
  "storage": {
    "filesystems": [
      {
        "mount": {
          "device": "/dev/disk/by-partlabel/ROOT",
          "format": "ext4",
          "create": {
            "force": true,
            "options": [
              "-L",
              "ROOT",
              "-U",
              "9aa5237a-ab6b-458b-a7e8-f25e2baef1a3"
            ]
          }
        }
      }
    ],
    "files": [
      {
        "filesystem": "root",
        "path": "/etc/coreos/update.conf",
        "contents": {
          "source": "data:,%0AREBOOT_STRATEGY%3D%22off%22",
          "verification": {}
        },
        "mode": 420
      }
    ]
  }
}

should result in /etc/flatcar/update.conf having REBOOT_STRATEGY="off".

Testing done

With the new image that also contains the change from flatcar/init#28 I was able to use the above Ignition config successfully, meaning that the symlink was set up and the update.conf file was created under the flatcar folder.

[margamanterola]

LGTM

[pothos]

I will include it in all maintenance branches.