I have just created a Buildroot configuration that allows building of SD card image for BPi M3. Buildroot_BPI_M3.zip (1.8 KB) After unpacking, just run the build.sh script on your Linux machine. Of course I don’t give warranty of any kind. It just works for me.
Good luck! Wojtek
PS. The configuration is also available on alt.sources: Thread in alt.sources abot BR configuration for BPI M3