USB image is not bootable anymore

I’ve tried to boot a USB image built from feature/15292-usb-image, both on bare metal and in a VM, and I see “This is not a bootable disk. Please insert a bootable floppy and press any key to try again …”. I guess one of the recent commits aimed at fixing reproducibility or GUIDs broke it.

Feature Branch: feature/15292-usb-image

Parent Task: #15293 (closed)

Original created by @intrigeri on 16149 (Redmine)