Provide a way to generate target-specific JSON files

[dylanmckay]

This repository knows about every single AVR device in existence (assuming that Atmel provides pack files for all chips).

Because of this, we can use it to generate target specification files for all microcontrollers, as they only differ by cpu.

I want to do this so that there is one central repository of JSON files (possibly in this repository, possibly not), so AVR-Rust users don't need to muck around with it.

[dylanmckay]

Could be cool to have a separate repository of all JSON files that can easily be embedded as a submodule.

Hopefully xargo supports target specifications that aren't in the repository root.