This requires the 3.0 version of PlatformIO. If the About page indicates version 2.x.x, then update to V3.x.x from the PlatformIO start page.
Run
platformio install platforms https://github.com/1bitsy/platform-1bitsy.git
For development, install the git repository by itself.
cd ~/.platformio/platforms/
git clone https://github.com/1bitsy/platform-1bitsy.git -o 1bitsy
Most of it is in builder/main.py. For the most part, everything is just setting up flags, which are appropriately named.
The "build recipes" start ~L#121 at BUILDERS=dict(.... These very closely match the Makefile for the system.
Conspicuously missing is any sort of a .map file at this time.
Upload stuff is currently left in from the stock STM32 project, pending the build process working right.