I am not a lawyer, so take this with a grain of salt but I don't see a legal issue with what you have described. In this context, derivative work would be modifying the smoothieware code or putting a non-trivial part of the smoothieware code into some other software. The software that runs on the PC that communicates with smoothieware would not be considered a derivative work of smoothieware unless you took some code from smoothieware to use in the PC software. If you are selling boards with unmodified smoothieware pre-flashed, then you need to provide users a link back to the smoothieware repo and provide users instructions on how to flash custom firmware if they ask.
See this page for an overview on GPLv3:
https://www.gnu.org/licenses/quick-guide-gplv3.en.html
GPLv3 software does not require the hardware which runs it to be open source, but it does require the user be able to update the GPLv3 software running on the hardware. This is referred to as the anti-Tivoization clause and is one of the main reasons for v3 of the GPL.