enable NixOS installer for powerpc64le

nixos/modules/installer/sd-card/sd-image-powerpc64le.nix

@@ -0,0 +1,49 @@
+# To build, use:
+# nix-build nixos -I nixos-config=nixos/modules/installer/sd-card/sd-image-powerpc64le.nix -A config.system.build.sdImage
+{ config, lib, pkgs, ... }:
+
+{
+  imports = [
+    ../../profiles/base.nix
+    ../../profiles/installation-device.nix
+    ./sd-image.nix
+  ];
+
+  boot.loader = {
+    # powerpc64le-linux typically uses petitboot
+    grub.enable = false;
+    generic-extlinux-compatible = {
+      # petitboot is not does not support all of the extlinux extensions to
+      # syslinux, but its parser is very forgiving; it essentially ignores
+      # whatever it doesn't understand.  See below for a filename adjustment.
+      enable = true;
+    };
+  };
+
+  boot.consoleLogLevel = lib.mkDefault 7;
+  boot.kernelParams = [ "console=hvc0" ];
+
+  sdImage = {
+    populateFirmwareCommands = "";
+    populateRootCommands = ''
+      mkdir -p ./files/boot
+      ${config.boot.loader.generic-extlinux-compatible.populateCmd} \
+        -c ${config.system.build.toplevel} \
+        -d ./files/boot
+    ''
+    # https://github.com/open-power/petitboot/blob/master/discover/syslinux-parser.c
+    # petitboot will look in these paths (plus all-caps versions of them):
+    #  /boot/syslinux/syslinux.cfg
+    #  /syslinux/syslinux.cfg
+    #  /syslinux.cfg
+    + ''
+      mv ./files/boot/extlinux ./files/boot/syslinux
+      mv ./files/boot/syslinux/extlinux.conf ./files/boot/syslinux/syslinux.cfg
+    ''
+    # petitboot does not support relative paths for LINUX or INITRD; it prepends
+    # a `/` when parsing these fields
+    + ''
+      sed -i 's_^\(\W\W*\(INITRD\|initrd\|LINUX\|linux\)\W\)\.\./_\1/boot/_' ./files/boot/syslinux/syslinux.cfg
+    '';
+  };
+}