initramfs: produce a single file in /nix/store rather than a directory

src/default.nix

@@ -15,7 +15,7 @@ let
 
       iasl_20180531 = self.nixpkgsOnBuildForBuild.callPackage ./coreboot/iasl_20180531 { };
       coreboot      = self.callPackage ./coreboot {
-        initramfs_image = "${self.initramfs}/initramfs.cpio";
+        initramfs_image = self.initramfs;
       };
       kernel        = self.callPackage ./kernel { };
       initramfs     = self.callPackage ./initramfs {

src/initramfs/default.nix

@@ -8,12 +8,11 @@
 }:
 
 nixpkgsOnBuildForHost.stdenv.mkDerivation {
-  name = "initramfs";
+  name = "initramfs.cpio";
   nativeBuildInputs = with nixpkgsOnBuildForBuild; [ findutils cpio ];
   srcs = [ ];
   dontUnpack = true;
   buildPhase = ''
-    mkdir -p $out
     mkdir build
     mkdir -p build/lib/modules
     BUILD=$(pwd)/build
@@ -25,7 +24,7 @@ nixpkgsOnBuildForHost.stdenv.mkDerivation {
     popd
     chmod -R u+w $BUILD
     pushd $BUILD
-    find . | cpio --create -H newc -R +0:+0 > $out/initramfs.cpio
+    find . | cpio --create -H newc -R +0:+0 > $out
     popd
   '';
   dontInstall = true;

src/platform/kevin/fit/default.nix

@@ -8,7 +8,7 @@
 }:
 let
   initramfs_lzma = runCommand "initramfs.lzma" { } ''
-    lzma < ${initramfs}/initramfs.cpio > $out;
+    lzma < ${initramfs} > $out;
   '';
 in stdenv.mkDerivation {
   name = "ownerboot-stage1-fit";