okay, i see what we need to get it to boot; A kind of usb-reset or refresh of the usb devices. it is not a problem of the bootloader, because a part of the image is already running bevor it stop.
1) take a usb hub with at minimum 2 ports
2) let the device boot until you see the message of #100 in the terminal
3) pull off the usb-stick and
4) wait a second
5) plug the usb-stick into the other port
6) the device continuses the booting and the image starts
edit: not working every time, but now i can see the problem, if the usb-stick is not lightning, than it does not work, if it lights it works. i seems like the usb driver are not correct, so that it is not every time correct initialized
edit2: if i change the uImage (from PKT) then it all the time starts booting (okay, not starting, because not the right initialisation, but they have solved this issue)