On linux-3.10, build in #! script support

Starting with 3.10, #! script handling can be built modularly (or not
at all). By default the nixpkgs builder sets everything modular, but
since our initird init is a #! script this creates a chicken-and-egg
problem on NixOS.

Signed-off-by: Shea Levy <shea@shealevy.com>