Modernize firmware builder to use PlatformIO

[amotl]

About

The firmware builder subsystem used to use very early Arduino 1.0.x already, and is currently effectively defunct. Within another iteration, it should be modernized to use PlatformIO. Contributions are welcome.

References

• Dissolve Makefile-based build tooling. Move on to PlatformIO. Thanks for all the fish! hiveeyes/arduino#58
• https://hiveeyes.org/docs/arduino/firmware-builder.html