nixos/sdImage: add postBuildCommands
nixos/sdImage: add postBuildCommands

This allows to perform `dd if= of=$img` after the image is built
which is handy to add e.g. uBoot SPL to the built image.

Instructions for some ARM boards sometimes contain this step
that needs to be performed manually, with this patch it can be
part of the nix file used to built the image.

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