Refactor 02-set-machine-id into an element

This script is needlessly duplicated across 3 elements. This change
consolidates those.

Assisted-By: Opencode - Claude Sonnet 4.6
Signed-off-by: Steve Baker <sbaker@redhat.com>
Change-Id: I8183931b1b962b7916265e3b5c3ea1175bd57c4d

diskimage_builder/elements/centos/element-deps

@@ -2,4 +2,5 @@ cache-url
 redhat-common
 rpm-distro
 source-repositories
+systemd-machine-id
 yum

diskimage_builder/elements/fedora/element-deps

@@ -3,4 +3,5 @@ dkms
 redhat-common
 rpm-distro
 source-repositories
+systemd-machine-id
 yum

diskimage_builder/elements/fedora/pre-install.d/02-set-machine-id

@@ -1,19 +0,0 @@
-#!/bin/bash
-
-if [ ${DIB_DEBUG_TRACE:-0} -gt 0 ]; then
-    set -x
-fi
-set -eu
-set -o pipefail
-
-# The systemd .spec file does this in %post [1] and it turns out that
-# (in what is possibly a bug [2]) that kernel install requires
-# /etc/machine-id.  This affects "fedora" (the image-based build) if
-# there is no systemd update to install ... with "fedora-minimal" we
-# are always installing into the chroot so, so %post always runs and
-# this is always run
-#
-# [1] https://src.fedoraproject.org/rpms/systemd/blob/master/f/systemd.spec
-# [2] https://bugzilla.redhat.com/show_bug.cgi?id=1486124
-
-systemd-machine-id-setup

diskimage_builder/elements/rhel/element-deps

@@ -3,4 +3,5 @@ redhat-common
 rhel-common
 rpm-distro
 source-repositories
+systemd-machine-id
 yum

diskimage_builder/elements/rhel/pre-install.d/02-set-machine-id

@@ -1,23 +0,0 @@
-#!/bin/bash
-
-if [ ${DIB_DEBUG_TRACE:-0} -gt 0 ]; then
-    set -x
-fi
-set -eu
-set -o pipefail
-
-# Set a machine-id.  The .qcow2 image doesn't have one, and sometimes
-# one will get made (if systemd is upgraded as part of the build
-# process) and sometimes not.  The problem is that certain kernel
-# install scripts bail silently without it; kernel packages end up
-# being installed but the initramfs etc isn't copied into place.
-#
-# Note this is cleared out in the sysprep role.
-#
-# See also:
-#  768c5e188c1b4bff01da14a49b96b51301db4c03 : similar thing for fedora
-#  https://bugzilla.redhat.com/show_bug.cgi?id=1737355 : upstream bug
-#  https://bugzilla.redhat.com/show_bug.cgi?id=1486124 : unresolved
-#   bug about kernel install requiring machine-id
-
-systemd-machine-id-setup

diskimage_builder/elements/systemd-machine-id/README.rst

@@ -0,0 +1,17 @@
+==================
+systemd-machine-id
+==================
+
+Initialises ``/etc/machine-id`` during image build by running
+``systemd-machine-id-setup`` in the pre-install phase.
+
+Image-based builds (e.g. from a qcow2 or tarball) may not have a
+machine-id present.  One may also be absent when systemd has not yet
+been upgraded as part of the build.  Certain kernel install scripts
+check for ``/etc/machine-id`` and bail silently when it is missing,
+leaving kernel packages installed but without an initramfs or kernel
+copied into place.
+
+The machine-id set here is a temporary build-time value.  It is
+cleared by the ``sysprep`` element so that each deployed instance
+receives a unique id on first boot.

diskimage_builder/elements/systemd-machine-id/pre-install.d/02-set-machine-id

@@ -6,7 +6,7 @@ fi
 set -eu
 set -o pipefail
 
-# Set a machine-id.  The .qcow2 image doesn't have one, and sometimes
+# Set a machine-id.  Image-based builds may not have one, and sometimes
 # one will get made (if systemd is upgraded as part of the build
 # process) and sometimes not.  The problem is that certain kernel
 # install scripts bail silently without it; kernel packages end up