Describe the bug
When using the os_prefix option, RPi3b+ boards are detected as RPi3b.

To reproduce
Copy dtbs, kernel, initrd, and cmdline.txt to a folder in the boot partition, and set os_config=<dir> in config.txt.

Expected behaviour
An RPI3b+ should be detected:

$ cat /proc/device-tree/compatible
raspberrypi,3-model-b-plus\0brcm,bcm2837\0

(this is the behaviour when os_prefix is not used)
Actual behaviour
An RPI3b is detected:

$ cat /proc/device-tree/compatible
raspberrypi,3-model-b\0brcm,bcm2837\0

One of the side effects that is produced is that the ethernet MAC is set to a randon number on each boot.

System

• Which model of Raspberry Pi? e.g. Pi3B+, PiZeroW
  Pi3B+
• Which OS and version (cat /etc/rpi-issue)?
  Ubuntu 22.04
• Which firmware version (vcgencmd version)?
  1.20211118
• Which kernel version (uname -a)?
  5.15.0

Logs
NA

Additional context
NA