feat: Add a nix expression

We provide a nix expression to build card10 firmware.

On Linux/Darwin with Nix and NixOS, this can be used as follows:

  $ git submodule update --init
  $ nix build
  $ ls result/
  result/bootloader:
  bootloader.elf

  result/epicardium:
  epicardium.elf

  result/pycardium:
  pycardium.elf  pycardium_epicardium.bin

This derivation can also be used for development:

  $ nix-shell
  $ meson --cross-file card10-cross.ini build/
  $ ninja -C build/ -j8
  $ # hack on things
  $ ninja -C build/ -j 8
  $ ls build/
  [ ... ]