overlay: add Conflicts=boot.automount

Otherwise, `systemd-gpt-auto-generator`'s generated `boot.automount`
will cause a funky mount structure:

```
├─/boot           systemd-1    autofs
│ └─/boot         /dev/vda1    ext4
│   └─/boot/efi   /dev/vda2    vfat
```

which then causes issues for us, especially around shutdown ordering.
See also the linked systemd issue.

I initially added `systemd.no_gpt=0` to the kargs in `image.yaml`,
because it's better to just *not* have generated mount units in the
first place. But I'm hesitant to resort to kargs yet, since it's
something we don't control as easily as a unit file. (And ideally, it's
only a temporary measure).

Closes: coreos/fedora-coreos-tracker#215

overlay/usr/lib/systemd/system/boot.mount

@@ -1,8 +1,9 @@
 [Unit]
 Description=Boot partition
 Documentation=https://github.com/coreos/fedora-coreos-config
-# This prevents the systemd-gpt-auto-generator from generating a mount unit for
-# /boot using the ESP.
+# This prevents the systemd-gpt-auto-generator generated mount unit
+# from starting: https://github.com/systemd/systemd/issues/13099
+Conflicts=boot.automount
 
 [Mount]
 What=/dev/disk/by-label/boot