src/image: rename parameter initramfs_image -> initramfs

This allows to have the argument automatically populated by
`callPackage`.

src/default.nix

@@ -27,9 +27,7 @@ let
 
       iasl_20180531 = final.nixpkgsOnBuildForBuild.callPackage ./coreboot/iasl_20180531 { };
       coreboot = final.callPackage ./coreboot { };
-      image = final.callPackage ./image {
-        initramfs_image = final.initramfs;
-      };
+      image         = final.callPackage ./image { };
       kernel        = final.callPackage ./kernel { };
       userspace     = final.callPackage ./userspace { };
       initramfs     = final.callPackage ./initramfs { };

src/image/default.nix

@@ -7,7 +7,7 @@
 , payload-compression-type ? "none"
 , payload-prefix           ? "prefix"
 , payload-name             ? "payload"
-, initramfs_image          # path to the initramfs `cpio` archive
+, initramfs                ? null # initramfs `cpio` archive
 , linux-command-line       ? null
 }:
 
@@ -34,8 +34,8 @@ nixpkgsOnBuildForHost.stdenv.mkDerivation {
       "-c${payload-compression-type}"
     ] ++ lib.optionals (linux-command-line != null) [
       "-C'${linux-command-line}'"
-    ] ++ lib.optionals (initramfs_image != null) [
-      "-I'${initramfs_image}'"
+    ] ++ lib.optionals (initramfs != null) [
+      "-I'${initramfs}'"
     ] ++ [
       "\n"
     ]);

src/platform/kevin/default.nix

@@ -78,7 +78,7 @@ in {
 
     image = prev.image.override {
       payload = "${final.fit}/Image.fit";
-      initramfs_image = null;  # it is part of the FIT image
+      initramfs = null;  # it is part of the FIT image
     };
 
     coreboot =