Recent versions of Raspbian have been auto-magically copying /boot/wpa_supplicacnt.conf to /etc/wpa_supplicant/wpa_supplicant.conf at boot as well as interpreting /boot/ssh to enable the SSH service to allow most OSes to do headless setup of Raspbian.
Try it out and report your findings here! You may be surprised at the ingenuity of Raspberry Pi engineers in spite of the fact that not every OS supports Linux filesystems 
This is how my script enables SSH.
I'll see if this works.
This works. It appears to copy /boot/wpa_supplicant.conf over /etc/wpa_supplicant/wpa_supplicant.conf and then delete /boot/wpa_supplicant.conf on boot. Is it possible to change the raspberry pi hostname with a similar method?
Trying it out now on a Raspberry Pi 3 B to go fully headless.
The machine I am using to edit/setup the SD card is Windoze.
I should be able to confirm once I have a suitable power supply on hand (need more amps).
Thanks for all the hard work in figuring out these nuances @jvanier, @Moors7 and @nrobinson2000 !
D7…uhm…Request GPIO chart… Confused. which analog and digital pin in pi?
Take a look at this link in the docs for pin definitions:
https://docs.particle.io/datasheets/raspberrypi-datasheet/#pin-definition
I finally received my power supply for my new RaspberryPi 3.
Since it is absolutely brand new, it has the latest version of Raspbian.
I can confirm that the behavior of /boot/wpa_supplicacnt.conf works exactly as you described.