build options?

I need to provide build options for my esp8266 compile, specifcally ATOMIC_FS_UPDATE. I thought I’d be able to put a build option in but the only docs I can find are for cli compiles with environment commands like export PLATFORMIO_BUILD_FLAGS=-DFOO. I am using vscode so I only know how to use Can someone tell me how to do this?

The main documentation page for the platformio.ini file shows you all configuration options. Including build_flags. Just write build_flags = -D ATOMIC_FS_UPDATE in your platformio.ini.