add "waitup", s6 readiness helper for network interfaces

run e.g. "waitup wlan0 10" to wait until wlan0 is operationally
up and running, and then send a newline to file descriptor 10

pkgs/waitup/default.nix

@@ -0,0 +1,31 @@
+{
+  lua5_3
+, netlink-lua
+, stdenv
+, makeWrapper
+}:
+let
+  lua = lua5_3;
+  netlink = netlink-lua.override {inherit lua;};
+  fennel = lua.pkgs.fennel;
+in stdenv.mkDerivation rec {
+  pname = "waitup";
+  version = "1";
+
+  buildInputs = [ lua netlink-lua ];
+  nativeBuildInputs = [ makeWrapper  fennel ];
+
+  src = ./.;
+
+  installPhase = ''
+    mkdir -p $out/bin $out/lib
+    fennel --compile  ${./waitup.fnl} > $out/lib/waitup.lua
+
+    makeWrapper ${lua}/bin/lua $out/bin/${pname} \
+      --prefix LUA_CPATH ";" ${netlink}/lib/lua/${lua.luaversion}/\?.so \
+      --add-flags $out/lib/waitup.lua
+  '';
+}
+
+# to use fennel.view,
+#  --prefix LUA_PATH ";" ${fennel}/share/lua/5.2/\?.lua \