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

lib/modules.nix

@@ -710,6 +710,7 @@ rec {
 
   mkOptionDefault = mkOverride 1500; # priority of option defaults
   mkDefault = mkOverride 1000; # used in config sections of non-user modules to set a default
+  mkImageMediaOverride = mkOverride 60; # image media profiles can be derived by inclusion into host config, hence needing to override host config, but do allow user to mkForce
   mkForce = mkOverride 50;
   mkVMOverride = mkOverride 10; # used by ‘nixos-rebuild build-vm’
 

nixos/modules/installer/cd-dvd/installation-cd-base.nix

@@ -30,11 +30,10 @@ with lib;
   # Add Memtest86+ to the CD.
   boot.loader.grub.memtest86.enable = true;
 
-  # On a fresh machine, before formatting, an installation
-  # media cannot assume an existing file system layout such
-  # as might be defined by the encapsulated host config.
-  swapDevices = mkOverride 60 [ ];
-  fileSystems = mkOverride 60 config.lib.isoFileSystems;
+  # An installation media cannot tolerate a host config defined file
+  # system layout on a fresh machine, before it has been formatted.
+  swapDevices = mkImageMediaOverride [ ];
+  fileSystems = mkImageMediaOverride config.lib.isoFileSystems;
 
   boot.postBootCommands = ''
     for o in $(</proc/cmdline); do

nixos/modules/installer/cd-dvd/iso-image.nix

@@ -614,14 +614,11 @@ in
     };
 
   };
-  # store them in lib so we can set the same fileSystems with a
-  # higher prio on installation media
-  # 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.
+
+  # store them in lib so we can mkImageMediaOverride the
+  # entire file system layout in installation media (only)
   config.lib.isoFileSystems = {
-    "/" = mkOverride 60
+    "/" = mkImageMediaOverride
       {
         fsType = "tmpfs";
         options = [ "mode=0755" ];
@@ -630,7 +627,7 @@ in
     # Note that /dev/root is a symlink to the actual root device
     # specified on the kernel command line, created in the stage 1
     # init script.
-    "/iso" = mkOverride 60
+    "/iso" = mkImageMediaOverride
       { device = "/dev/root";
         neededForBoot = true;
         noCheck = true;
@@ -638,20 +635,20 @@ in
 
     # In stage 1, mount a tmpfs on top of /nix/store (the squashfs
     # image) to make this a live CD.
-    "/nix/.ro-store" = mkOverride 60
+    "/nix/.ro-store" = mkImageMediaOverride
       { fsType = "squashfs";
         device = "/iso/nix-store.squashfs";
         options = [ "loop" ];
         neededForBoot = true;
       };
 
-    "/nix/.rw-store" = mkOverride 60
+    "/nix/.rw-store" = mkImageMediaOverride
       { fsType = "tmpfs";
         options = [ "mode=0755" ];
         neededForBoot = true;
       };
 
-    "/nix/store" = mkOverride 60
+    "/nix/store" = mkImageMediaOverride
       { fsType = "overlay";
         device = "overlay";
         options = [

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 = [