os-release: Add CODENAME field

Debian and their derivatives (Ubuntu, Trisquel, etc.) use a code name
for their repositories. Thus record the code name in os-release for
processing.

Closes systemd/systemd#3429

man/os-release.xml

@@ -309,6 +309,22 @@
         </para></listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><varname>CODENAME=</varname></term>
+
+        <listitem><para>
+        A lower-case string (no spaces or other characters outside of
+        0–9, a–z, ".", "_" and "-") identifying the operating system
+        release code name, excluding any OS name information or
+        release version, and suitable for processing by scripts or
+        usage in generated filenames. This field is optional and may
+        not be implemented on all systems.
+        Examples:
+        <literal>CODENAME=buster</literal>,
+        <literal>CODENAME=xenial</literal>
+        </para></listitem>
+      </varlistentry>
+
     </variablelist>
 
     <para>If you are reading this file from C code or a shell script