This repository contains an example project using code generated using Atmel Start as a library in a PlatformIO project. The generated code is in lib/AtmelStart/generated, a little python script library.json.py reads the generated gcc/Makefile and creates library.json in the format required by PlatformIO.
Note: I am not using this approach anymore, as I realised it was more practical to just cut the middleman and to directly create a CMake toolchain from the generated Makefile. See here for more info.