The rotation is probably in your driver software. For me, with nvidia, I right click on the desktop and in the nvidia control panel there is a "rotate display" section.
In there I can select the Rift and, confusingly, select "portrait" for the orientation.
At that point the job is done. As a quick test before starting LFS, drag a window off to the right of your desktop onto the rift and look with one eye into the lens - you should see the window in your rift in the expected orientation.