Motivation for this change

This adds sd-mux-ctrl which is a tool for controlling multiple sd-mux devices for automated testing of single board computers: https://wiki.tizen.org/Laboratory

I figured this might be useful to others who are working on single-board computers (for example nixos: https://nixos.wiki/wiki/NixOS_on_ARM/Raspberry_Pi) and require automated testing around these systems.

I have been using sd-mux-ctrl with SDWire hardware. This board is based on sd-mux devices minus any power switch or USB switching. Essentially it enables you to flash the sdcard without having to take the card out of the board. The workflow ends up being something along the lines of:

sudo sd-mux-ctrl --device-serial=odroid_u3_1 --ts
... - do the flashing things (dd)
sudo sd-mux-ctrl --device-serial=odroid_u3_1 --dut

ts = test server (i.e. laptop, workstation)
dut = device under test (i.e. rpi, Odroid, etc.)

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.