Hi,
I reply to myself as I found the answer after a long grep on the whole drive. I'll put the answer here in case someone else stumble on this in the future. You need to edit
/boot/syslinux/syslinux.cfg
I don't know why but I couldn't see this file (or the content of the whole /boot folder for that matter) with the LiveCD, I had to swap the monitor with an LCD one.
Anyway, case closed for this one.