Create an IMG file from Deploy folder


How to create an img(img.xz) file like the latest build given in

I have used the Debian 8 (from Robert Nelson) with a custom 3.14.17-bone8 kernel. I am able to install the custom built kernel file to sd card from the bb-kernel/deploy/ folder whereas I need to package and provide as single img file. Is there an instructions to create an img file.