Add firmware build success message
Add firmware build success message

This adds a short message for what to do next after running `mix
firmware`. The goal is to have a simple reminder for what to do next.

It looks like this:

```sh
Building /Users/fhunleth/git/nerves-project/nerves_examples/blinky/_build/rpi0_dev/nerves/images/blinky.fw...
Firmware built successfully! 🎉

Now you may install it to a MicroSD card using `mix burn` or upload it
to a device with `mix upload` or `mix firmware.gen.script`+`./upload.sh`.
```

nerves-project/nerves,nerves-project/nerves