Fix GPT backup header of the USB image during boot

Originally created by @segfault on #15316 (Redmine)

In the GPT layout, a backup of the partition table is located in the last sector of the disk. This is not the case after the USB image is copied to a device, because the image (most probably) has a different size than the device. The plan is to fix this in initramfs during the first boot of the device.

Feature Branch: segfault:feature/15292-usb-image

Parent Task: #15293 (closed)