support jffs2, with initramfs

the jffs2 filesystem contains only /nix/store and a script which is
run in early init (initramfs) and is responsible for recreating
"traditional" directories (/bin /etc/**/* /var &c) based on the
configuration.

this is tested only in qemu so far and could use some cleanup

modules/initramfs.nix

@@ -0,0 +1,79 @@
+{
+  config
+, pkgs
+, lib
+, ...
+}:
+let
+#  inherit (lib) mkOption types concatStringsSep;
+  inherit (pkgs) runCommand callPackage writeText;
+in
+{
+  config = {
+    kernel.config.BLK_DEV_INITRD = "y";
+
+    outputs = {
+      initramfs =
+        let
+          bb = pkgs.busybox.override {
+            enableStatic = true;
+            enableMinimal = true;
+            enableAppletSymlinks  = false;
+            extraConfig  = ''
+              CONFIG_ASH y
+              CONFIG_LS y
+              CONFIG_LN y
+              CONFIG_CAT y
+              CONFIG_MOUNT y
+              CONFIG_PRINTF y
+              CONFIG_FEATURE_MOUNT_FLAGS y
+              CONFIG_FEATURE_MOUNT_VERBOSE y
+              CONFIG_ECHO y
+              CONFIG_CHROOT y
+              CONFIG_CHMOD y
+              CONFIG_MKDIR y
+              CONFIG_MKNOD y
+              CONFIG_SH_IS_ASH y
+              CONFIG_BASH_IS_NONE y
+            '';
+          };
+          slashinit = pkgs.writeScript "init" ''
+            #!/bin/sh
+            exec >/dev/console
+            echo IT MOVES
+            mount -t proc none /proc
+            mount -t jffs2 mtd0 /target/persist
+            mount -o bind /target/persist/nix /target/nix
+            sh /target/persist/activate /target
+            cd /target
+            mount -o bind /target /
+            exec chroot . /bin/init "$@"
+          '';
+          refs = pkgs.writeReferencesToFile bb;
+          gen_init_cpio = pkgs.pkgsBuildBuild.gen_init_cpio;
+        in runCommand "initramfs" {} ''
+          cat << SPECIALS | ${gen_init_cpio}/bin/gen_init_cpio /dev/stdin > $out
+          dir /proc 0755 0 0
+          dir /dev 0755 0 0
+          nod /dev/mtdblock0 0600 0 0 b 31 0
+          dir /target 0755 0 0
+          dir /target/persist 0755 0 0
+          dir /target/nix 0755 0 0
+          nod /dev/console 0600 0 0 c 5 1
+          dir /bin 0755 0 0
+          file /bin/busybox ${bb}/bin/busybox 0755 0 0
+          slink /bin/sh /bin/busybox 0755 0 0
+          slink /bin/echo /bin/busybox 0755 0 0
+          slink /bin/mount /bin/busybox 0755 0 0
+          slink /bin/chroot /bin/busybox 0755 0 0
+          slink /bin/chmod /bin/busybox 0755 0 0
+          slink /bin/ln /bin/busybox 0755 0 0
+          slink /bin/mkdir /bin/busybox 0755 0 0
+          slink /bin/mknod /bin/busybox 0755 0 0
+          slink /bin/printf /bin/busybox 0755 0 0
+          file /init ${slashinit} 0755 0 0
+          SPECIALS
+        '';
+    };
+  };
+}

modules/jffs2.nix

@@ -0,0 +1,49 @@
+{
+  config
+, pkgs
+, lib
+, ...
+}:
+let
+  inherit (lib) mkOption types concatStringsSep;
+  inherit (pkgs) liminix callPackage writeText closureInfo;
+in
+{
+  imports = [
+    ./initramfs.nix
+  ];
+  config = {
+    kernel.config.JFFS2_FS = "y";
+    outputs = rec {
+      systemConfiguration =
+        let inherit (pkgs.pkgsBuildBuild) systemconfig;
+        in systemconfig config.filesystem.contents;
+      jffs2fs =
+        let
+          inherit (pkgs.pkgsBuildBuild) runCommand systemconfig mtdutils;
+          sysconf = systemConfiguration;
+        in runCommand "make-jffs2" {
+          depsBuildBuild = [ mtdutils ];
+        } ''
+          mkdir -p $TMPDIR/empty/nix/store/
+          cp ${sysconf}/activate  $TMPDIR/empty/activate
+          pkgClosure=${closureInfo { rootPaths = [ sysconf ]; }}
+          cp $pkgClosure/registration nix-path-registration
+          grafts=$(sed < $pkgClosure/store-paths 's/^\(.*\)$/--graft \1:\1/g')
+          mkfs.jffs2 --pad  --big-endian --root $TMPDIR/empty --output $out  $grafts  --verbose
+        '';
+      jffs2boot =
+        let o = config.outputs; in
+        pkgs.runCommand "jffs2boot" {} ''
+          mkdir $out
+          cd $out
+          ln -s ${o.jffs2fs} rootfs
+          ln -s ${o.kernel} vmlinux
+          ln -s ${o.manifest} manifest
+          ln -s ${o.initramfs} initramfs
+       '';
+
+
+    };
+  };
+}