Bingo !
it boots without the usb key now 
So to unbrick the device, i flashed this image : on an usb stick with Rufus.
inserted it in one of the usb port, and pluged in the power cable.
at the F12 prompt, selected my usb key. and then it booted.
after a few seconds, once the text is no longer scrolling on the screen. just push enter.
and run the command :
./installer -d sdb -p
once finished, type reboot, and remove the usbkey.
now i'm with the unofficial OpenWrt...at least.