According to Raspberry Pi documentation, the RPi 5 First Stage Bootloader
loads the linux kernel instead of loading the Second Stage Bootloader
(start.elf):
https://www.raspberrypi.com/documentation/computers/raspberry-pi.html#differences-on-raspberry-pi-5
Fixes: 8b63d9a ("bcm27xx: add new bcm2712 subtarget")
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>