nixos filesystems: unify early filesystems handling

A new internal config option `fileSystems.<name>.early` is added to indicate
that the filesystem needs to be loaded very early (i.e. in initrd). They are
transformed to a shell script in `system.build.earlyMountScript` with calls to
an undefined `specialMount` function, which is expected to be caller-specific.
This option is used by stage-1, stage-2 and activation script to set up and
remount those filesystems.  Options for them are updated according to systemd
defaults.

nixos/modules/system/activation/activation-script.nix

@@ -154,9 +154,15 @@ in
 
     system.activationScripts.tmpfs =
       ''
-        ${pkgs.utillinux}/bin/mount -o "remount,size=${config.boot.devSize}" none /dev
-        ${pkgs.utillinux}/bin/mount -o "remount,size=${config.boot.devShmSize}" none /dev/shm
-        ${pkgs.utillinux}/bin/mount -o "remount,size=${config.boot.runSize}" none /run
+        specialMount() {
+          local device="$1"
+          local mountPoint="$2"
+          local options="$3"
+          local fsType="$4"
+
+          ${pkgs.utillinux}/bin/mount -t "$fsType" -o "remount,$options" "$device" "$mountPoint"
+        }
+        source ${config.system.build.earlyMountScript}
       '';
 
   };