overriden. Some defaults should not be evaluated because they cause
unnecessary evaluation or might cause evaluation errors
(e.g. assertions). For instance, the value of the
`boot.kernelPackages' option is an attribute set. Some packages in
that set might not evaluate (e.g. AUFS2 on kernels that don't have
the AUFS2 patch), and in any case it slows down nixos-rebuild to
evaluate lots of packages that are not actually used in the
configuration.
svn path=/nixpkgs/trunk/; revision=22043
1ba3165b4e