core: consult credentials for machine ID to use for host

Let's hook up one more thing with credentials: the machine ID to use
when none is initialized yet.

This requires some reordering of initialization steps in PID 1: we need
to import credentials first, and only then initialize the machine ID.

man/systemd.system-credentials.xml

@@ -208,6 +208,15 @@
           <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>.</para>
         </listitem>
       </varlistentry>
+
+      <varlistentry>
+        <term><varname>system.machine_id</varname></term>
+        <listitem>
+          <para>Takes a 128bit ID to initialize the machine ID from (if it is not set yet). Interpreted by
+          the service manager (PID 1). For details see
+          <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>.</para>
+        </listitem>
+      </varlistentry>
     </variablelist>
   </refsect1>