nixos/netboot: add squashfsCompression option

This is copied from isoImage.squashfsCompression.  It's useful to be
able to customise, as iteration cycles are very slow with xz, and
subjectively systems booted with less efficiently compressed squashfs
stores appear to have faster reads (although I didn't test that
scientificly so there could be other factors).

nixos/modules/installer/netboot/netboot.nix

@@ -8,6 +8,20 @@ with lib;
 {
   options = {
 
+    netboot.squashfsCompression = mkOption {
+      default = with pkgs.stdenv.hostPlatform; "xz -Xdict-size 100% "
+                + lib.optionalString isx86 "-Xbcj x86"
+                # Untested but should also reduce size for these platforms
+                + lib.optionalString isAarch "-Xbcj arm"
+                + lib.optionalString (isPower && is32bit && isBigEndian) "-Xbcj powerpc"
+                + lib.optionalString (isSparc) "-Xbcj sparc";
+      description = lib.mdDoc ''
+        Compression settings to use for the squashfs nix store.
+      '';
+      example = "zstd -Xcompression-level 6";
+      type = types.str;
+    };
+
     netboot.storeContents = mkOption {
       example = literalExpression "[ pkgs.stdenv ]";
       description = lib.mdDoc ''
@@ -77,6 +91,7 @@ with lib;
     # Create the squashfs image that contains the Nix store.
     system.build.squashfsStore = pkgs.callPackage ../../../lib/make-squashfs.nix {
       storeContents = config.netboot.storeContents;
+      comp = config.netboot.squashfsCompression;
     };