In fact you don't have to do anything but stop being stupid like me. As explained here: https://raspberrypi.stackexchange.com/questions/14953/is-it-possible-to-boot-the-raspberry-pi-in-secure-mode
When the NS
bit of the SCR
register is 0, it means that you are in Secure Mode, which is the case at boot on my Raspberry. I had the meaning of this bit backwards, sorry !