I have made some tests with SeaBIOS and XHCI and found, that on my board (which is
a USB3.0 hub won't work. For those who are familar with SeaBIOS I have attached two
logfiles from SeaBIOS. One of them is with the USB3.0 flash drive directly connected to
one of the root ports of SoC
(in which case the boot process starts from the flash drive) and the other is with the
same flash drive but a USB3.0 hub in between
(I have used a hub from Delock #62534,
). In the hub-case, I see
"No bootable device" screen and no boot will happen.
Maybe someone more experienced can see the cause from the logs.