default.nix: init [MANUAL MERGE]

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/
[ ... ]