I assume by
From a previous discussion, I successfully put a standard Yocto image on my SD card which allows me to use my Gen2 Galileo as an Arduino.
you meant the one described at SD Card Setup and Sketch Persistent for Boards and Kits, right?
So now I want to experiment with reading a data file. So do I simply create a comma delimited text file on the same SD card along side the other five Yocto image files or what?
If you want to read that file from a sketch then the answer is yes. Take into account, however, you will see the file in the place where the SD card filesystem is mounted in this Linux, which usually is
i.e. if you create a file test.txt in the SD card it is going to be readable from a sketch as
as a regular file, actually, using (Linux) regular file operations.
The discussion I was referring to was one of my own Getting wifi sketch to recognize Galileo wifi.
So yes everything worked fine. I put a blank test.txt file on the SD card and compiled the ReadWrite.ino file without any changes.