ofono: fix config loading directory properly

It should be loaded from /etc but installed to $out/etc so that people can see what they should place to /etc.

pkgs/tools/networking/ofono/default.nix

@@ -44,17 +44,13 @@ stdenv.mkDerivation rec {
     "--with-dbusconfdir=${placeholder "out"}/share"
     "--with-systemdunitdir=${placeholder "out"}/lib/systemd/system"
     "--enable-external-ell"
+    "--sysconfdir=/etc"
   ];
 
   installFlags = [
-    "SYSCONFDIR=${placeholder "out"}/etc"
+    "sysconfdir=${placeholder "out"}/etc"
   ];
 
-  postInstall = ''
-    rm -r $out/etc/ofono
-    ln -s /etc/ofono $out/etc/ofono
-  '';
-
   enableParallelBuilding = true;
   enableParallelChecking = false;