TL;DR* Here's what the license entails:
1. Anyone can copy, modify and distribute this software.
2. You have to include the license and copyright notice with each and every distribution.
3. You can use this software privately.
4. You can use this software for commercial purposes.
5. If you dare build your business solely from this code, you risk open-sourcing the whole code base.
6. If you modify it, you have to indicate changes made to the code.
7. Any modifications of this code base MUST be distributed with the same license, GPLv3.
8. This software is provided without warranty.
9. The software author or license can not be held liable for any damages inflicted by the software.
More information on about the LICENSE can be found here
@MasterKia , I don't know how one defines the phrase "intimate enough". In my case the only communication between the Firmware and the program running on the additional microprocessor is commands from the Firmware to the additional microprocessor telling it to perform some operation (e.g. turning a light on having it drive a stepper motor), and for the additional microprocessor to respond with a "DONE" or "FAILED" message back to the Firmware. No "complex data structures" are communicated.