sd-image: Add option to control sd image expansion on boot.
sd-image: Add option to control sd image expansion on boot.

This is supeer useful to allow the normal sd-image code to be used by
someone who wants to setup multiple partitions with a sd-image.

Currently I'm manually copying the sd-image file and modifying it
instead.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs