[giorgio130]

I was worried that the u-boot was not stored on the sd-card, so flashing a non-working bootloader could brick the device. It doesn't seem to be the case:

UBOOT=$1
WAVEFORM=$2
KERNEL=$3
DEVICE=$4

if [ -e $UBOOT ]; then
	echo Upgrading U-Boot $UBOOT
	dd if=$UBOOT of=/dev/$DEVICE bs=1K seek=1 skip=1
	if [ $? != 0 ] ; then
		exit 1
	fi
	sync
	sync
	echo Done upgrading U-Boot
fi

if [ -e $WAVEFORM ]; then
	echo Upgrading Waveform $WAVEFORM
	dd if=$WAVEFORM.header of=/dev/$DEVICE bs=512 seek=14335
	dd if=$WAVEFORM of=/dev/$DEVICE bs=512 seek=14336
	if [ $? != 0 ] ; then
		exit 1
	fi
	sync
	sync
	echo Done upgrading Waveform
fi

if [ -e $KERNEL ]; then
	echo Upgrading kernel... $KERNEL
	dd if=$KERNEL of=/dev/$DEVICE bs=512 seek=2048
	if [ $? != 0 ]; then
		exit 1
	fi
	sync
	sync
	echo Done upgrading kernel...
fi

this is the code that flashes it. As you can see the location to where it is flashed is the 4th argument. And this is the only place where the function is called:
/etc/init.d/upgrade-generic.sh $UBOOT $WAVEFORM $KERNEL mmcblk0
in upgrade-wifi.sh.
So everything seems safe... I just have to gather enough courage now