makeInitrd: don't assume uImage => arm

mips for example might use uImages too

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

···
 , prepend ? []
 , lib
 , makeUInitrd ? stdenvNoCC.hostPlatform.platform.kernelTarget == "uImage"
+, uInitrdArch ? stdenvNoCC.hostPlatform.parsed.cpu.family
 }:
 let
   # !!! Move this into a public lib function, it is probably useful for others
···
     lib.concatStringsSep "-" (filter (x: !(isList x)) (split "[^a-zA-Z0-9_=.?-]+" x));
 
 in stdenvNoCC.mkDerivation rec {
-  inherit name makeUInitrd;
+  inherit name makeUInitrd uInitrdArch;
 
   builder = ./make-initrd.sh;
 

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

···
 
 if [ -n "$makeUInitrd" ]; then
     mv $out/initrd $out/initrd.gz
-    mkimage -A arm -O linux -T ramdisk -C gzip -d $out/initrd.gz $out/initrd
+    mkimage -A $uInitrdArch -O linux -T ramdisk -C gzip -d $out/initrd.gz $out/initrd
 fi