Trivial network-controlled plugs

Instructions

Build the firmware with default.nix:

• nix-build
• This uses cargo-vendor to bundle dependencies, so that unstable versions from git can be used.
• Run result/bin/flash-tnetplug to flash a devboard with OpenOCD and quit.

Development environment with shell.nix:

• nix-shell
• Spawning openocd, the devboard should be connected already.
• Instructions (cargo run --release) are printed.