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

pkgs/default.nix

@@ -12,6 +12,7 @@
     };
   };
   writeAshScript = callPackage ./write-ash-script {};
+  systemconfig = callPackage ./systemconfig {};
   s6-init-bin =  callPackage ./s6-init-bin {};
   s6-rc-database = callPackage ./s6-rc-database {};
   mips-vm = callPackage ./mips-vm {};