kernel-install: respect $MACHINE_ID and ignore /etc/machine-id if on tmpfs

Confer https://github.com/systemd/systemd/pull/19006#issuecomment-800234022:
  On some systems it's the admin's explicit choice not to to have the
  machine ID leak into the ESP
  On some systems the machine ID is transient, generated at every boot,
  and hence should not be written to the ESP

man/kernel-install.xml

@@ -161,8 +161,12 @@
 
   <refsect1>
     <title>Environment variables</title>
+
     <para>If <option>--verbose</option> is used, <varname>$KERNEL_INSTALL_VERBOSE=1</varname> will be set for
     the plugins. They may output additional logs in this case.</para>
+
+    <para>If <varname>MACHINE_ID=</varname> is set and not empty, it will be used as <replaceable>MACHINE-ID</replaceable>,
+    overriding any automatic detection attempts.  The value must be a valid machine ID (32 hexadecimal characters).</para>
   </refsect1>
 
   <refsect1>
@@ -214,9 +218,9 @@
         </term>
           <listitem>
             <para>The content of this file specifies the machine identification
-            <replaceable>MACHINE-ID</replaceable>.  If <filename>$BOOT/Default</filename> exists,
-            or <filename>/etc/machine-id</filename> doesn't, <command>kernel-install</command>
-            will use the literal <literal>Default</literal> as the machine ID instead.</para>
+            <replaceable>MACHINE-ID</replaceable>. If <filename>/etc/machine-id</filename>
+            cannot be read or is temporary (backed by a file on <constant>tmpfs</constant>),
+            <command>kernel-install</command> will use <literal>Default</literal> instead.</para>
           </listitem>
       </varlistentry>
       <varlistentry>