This is my automated solution for configuring a Raspbian SD card for headless use with Particle.
Essentially the script is a wizard that guides the user through the steps on my blog.
This script requires a Linux environment, preferably Ubuntu-based, so that the SD card is mounted in
After flashing your SD card with Raspbian just run the script to setup your Pi for headless use:
$ bash <( curl -sL https://raw.githubusercontent.com/nrobinson2000/pi-setup/master/pi-setup.sh)