problem is that configuration values below a mkIf are evaluated
strictly even if the condition is false. Thus "${luksRoot}" causes
an evaluation error. As a workaround, use the empty string instead
of `null' as the default value. However, we should really fix the
laziness of mkIf. It's likely that NixOS evaluation would be much
faster if it didn't have to evaluate disabled configuration values.
svn path=/nixos/trunk/; revision=24477
