build: factor ext2 filenames

Factor them in own variables to avoid repeating them in following
commits.

This is just refactoring, no behaviour changes.

src/mode_build.ml

@@ -59,6 +59,10 @@ and string_of_file_content = function
   | Excludefiles -> "excludefiles"
   | Empty -> "empty"
 
+let kernel_filename = "kernel"
+and appliance_filename = "root"
+and initrd_filename = "initrd"
+
 let rec build debug
     (copy_kernel, format, host_cpu,
      packager_config, tmpdir, use_installed, size,
@@ -223,9 +227,9 @@ let rec build debug
     Format_chroot.build_chroot debug files outputdir packagelist_file
 
   | Ext2 ->
-    let kernel = outputdir // "kernel"
-    and appliance = outputdir // "root"
-    and initrd = outputdir // "initrd" in
+    let kernel = outputdir // kernel_filename
+    and appliance = outputdir // appliance_filename
+    and initrd = outputdir // initrd_filename in
     let kernel_version, modpath =
       Format_ext2_kernel.build_kernel debug host_cpu copy_kernel kernel in
     Format_ext2.build_ext2 debug basedir files modpath kernel_version