lib/modules: add mkImageMediaOverride

so the underlaying use case of the preceding commit is so
generic, that we gain a lot in reasoning to give it an
appropriate name.

As the comment states:
image media needs to override host config short of mkForce

1 files changed, 4 insertions, 8 deletions

diff --git a/nixos/modules/installer/netboot/netboot.nix b/nixos/modules/installer/netboot/netboot.nix
index adcaf9d1a335..28b6c39b29df 100644
--- a/nixos/modules/installer/netboot/netboot.nix
+++ b/nixos/modules/installer/netboot/netboot.nix
@@ -29,31 +29,27 @@ with lib;
           then []
           else [ pkgs.grub2 pkgs.syslinux ]);
 
-    # This module is often over-layed onto an existing host config
-    # that defines `fileSystems`. We use mkOverride 60 to override
-    # standard values, but at the same time leave room for mkForce
-    # values targeted at the image build.
-    fileSystems."/" = mkOverride 60
+    fileSystems."/" = mkImageMediaOverride
       { fsType = "tmpfs";
         options = [ "mode=0755" ];
       };
 
     # In stage 1, mount a tmpfs on top of /nix/store (the squashfs
     # image) to make this a live CD.
-    fileSystems."/nix/.ro-store" = mkOverride 60
+    fileSystems."/nix/.ro-store" = mkImageMediaOverride
       { fsType = "squashfs";
         device = "../nix-store.squashfs";
         options = [ "loop" ];
         neededForBoot = true;
       };
 
-    fileSystems."/nix/.rw-store" = mkOverride 60
+    fileSystems."/nix/.rw-store" = mkImageMediaOverride
       { fsType = "tmpfs";
         options = [ "mode=0755" ];
         neededForBoot = true;
       };
 
-    fileSystems."/nix/store" = mkOverride 60
+    fileSystems."/nix/store" = mkImageMediaOverride
       { fsType = "overlay";
         device = "overlay";
         options = [