[settings] Expose MAC address setting for general use

src/include/gpxe/settings.h

@@ -216,6 +216,7 @@ extern struct setting priority_setting __setting;
 extern struct setting bios_drive_setting __setting;
 extern struct setting uuid_setting __setting;
 extern struct setting next_server_setting __setting;
+extern struct setting mac_setting __setting;
 
 /**
  * Initialise a settings block