adding maybeEnv. This will replace from-env.nix in nixos and will be reused in copySystemConfiguration

From trunk@21353

svn path=/nixpkgs/branches/mass-update-01/; revision=21389

pkgs/lib/misc.nix

@@ -9,6 +9,11 @@ with import ./strings.nix;
 
 rec {
 
+  # returns default if env var is not set
+  maybeEnv = name: default:
+    let value = builtins.getEnv name; in
+    if value == "" then default else value;
+
   defaultMergeArg = x : y: if builtins.isAttrs y then
     y
   else