There are a few missing steps in the Onion How To
Once you have used git, open a terminal window in the source folder.
You then need to download the packages as configured in the feeds.conf folder. This is done by
/scripts/feeds update -a
You can then install all the packages
/scripts/feeds update -a
When you execute make menuconfig, you will now find the libugpio option in Library, select it to be included in the build