standard: convert apply updates enablement to a preset

The way it was done before with a symlink in
/usr/lib/systemd/system/default.target.wants/bootc-fetch-apply-updates.timer
meant that users (or derivative builds) can't easily disable
this service without masking it.

The docs at [1] would work for the current boot because of the
`--now` but the next boot the timer would be running again.

[1] e7d868de85/modules/ROOT/pages/manual-rollbacks.adoc

standard/autoupdates.yaml

@@ -1,8 +0,0 @@
-# Enable automatic updates by default
-postprocess:
-  - |
-    #!/usr/bin/env bash
-    set -xeuo pipefail
-    target=/usr/lib/systemd/system/default.target.wants
-    mkdir -p $target
-    ln -s ../bootc-fetch-apply-updates.timer $target

standard/manifest.yaml

@@ -9,13 +9,13 @@ recommends: true
 
 include:
   - ../minimal-plus/manifest.yaml
-  - autoupdates.yaml
   - networking-tools.yaml
   - system-configuration.yaml
   - coreos-user-experience.yaml
   - persistent-journal.yaml
   - initramfs-full.yaml
   - generic-growfs.yaml
+  - systemd-presets.yaml
 
 packages:
   # Include and set the default editor

standard/systemd-presets.yaml

@@ -0,0 +1,14 @@
+# Postprocessing relating to systemd presets on the system.
+#
+# Note this relies on the `systemctl preset-all` run from
+# minimal/systemd-presets.yaml running after this postprocess.
+postprocess:
+  # Enable automatic updates by default
+  - |
+    #!/bin/bash
+    set -xeuo pipefail
+    # Enable automatic updates by default
+    cat <<EOF > usr/lib/systemd/system-preset/86-bootc-standard.preset
+    # Enable automatic updates by default
+    enable bootc-fetch-apply-updates.timer
+    EOF

tests/rootfs/cases/bootc-fetch-apply-updates

@@ -0,0 +1,16 @@
+#!/bin/bash
+set -xeuo pipefail
+
+# Right now the `systemctl preset-all` runs via a postprocess in
+# minimal/systemd-presets.yaml but the preset for the timer is only
+# set up in a postprocess in standard/systemd-presets.yaml.
+#
+# This test ensures that the ordering works out such that the preset
+# from standard got applied.
+#
+# Verify if the preset file exists (only on standard) that the timer
+# is enabled. Right now the `s
+if [ -f /usr/lib/systemd/system-preset/86-bootc-standard.preset ]; then
+    # Will return success (0) if enabled or failure (1) if not
+    systemctl is-enabled bootc-fetch-apply-updates.timer
+fi