makeInitrd: Support prepending other initrds

pkgs/build-support/kernel/make-initrd.nix

@@ -12,7 +12,7 @@
 # `contents = {object = ...; symlink = /init;}' is a typical
 # argument.
 
-{stdenv, perl, perlArchiveCpio, cpio, contents, ubootChooser, compressor}:
+{ stdenv, perl, perlArchiveCpio, cpio, contents, ubootChooser, compressor, prepend }:
 
 let
   inputsFun = ubootName : [perl cpio perlArchiveCpio ]
@@ -41,5 +41,5 @@ stdenv.mkDerivation {
     nativeBuildInputs = inputsFun stdenv.cross.platform.uboot;
     makeUInitrd = makeUInitrdFun stdenv.cross.platform.uboot;
   };
-  inherit compressor;
+  inherit compressor prepend;
 }

pkgs/build-support/kernel/make-initrd.sh

@@ -36,7 +36,10 @@ storePaths=$(perl $pathsFromGraph closure-*)
 
 # Put the closure in a gzipped cpio archive.
 mkdir -p $out
-(cd root && find * -print0 | cpio -o -H newc --null | perl $cpioClean | $compressor > $out/initrd)
+for PREP in $prepend; do
+  cat $PREP >> $out/initrd
+done
+(cd root && find * -print0 | cpio -o -H newc --null | perl $cpioClean | $compressor >> $out/initrd)
 
 if [ -n "$makeUInitrd" ]; then
     mv $out/initrd $out/initrd.gz