scripts: Add `imgtool.nix`
scripts: Add `imgtool.nix`

Nix environment for working with `imgtool.py`

Signed-off-by: Rajiv Ranganath <4acda8d1ee7b6cc93e786e53408b868c7be3fe76@atihita.com>

utzig/mcuboot,utzig/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,tamban01/mcuboot,tamban01/mcuboot,tamban01/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,utzig/mcuboot,utzig/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,utzig/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot