IPFire on the Raspberry PI 3 b+ does not boot from USB Stick

Hello,

I am trying to install/boot IPFire on my Raspberry PI 3b+ from USB-Stick But it does not work.

To prepare the whole thing I have activated the USB boot mode with the parameter in the terminal:
echo program_usb_boot_mode=1 | sudo tee -a /boot/config.txt

Then I checked if the parameter was set correctly with:
vcgencmd otp_dump | grep 17:

I then removed the SD card from the Raspberry and connected the flashed USB stick.
I used the following image and ran it with Rufus.

arm - Flash Image
232MB
SHA256: 124b02827f1f370837ed264fd3cfedd3e7b387a287f0b74a669f34476a2d3b58

Now I have started my Raspberry and it comes only

missing environment variable : bootfile
Retrieving fie: pxelinux.cfg/000
No ethernet found.
missing environment variable : bootfile
Retrieving fie: pxelinux.cfg/00
No ethernet found.
missing environment variable : bootfile
Retrieving fie: pxelinux.cfg/0
No ethernet found.
missing environment variable : bootfile
Retrieving fie: pxelinux.cfg/default-arm-bcm283x
No ethernet found.
missing environment variable : bootfile
Retrieving fie: pxelinux.cfg/default-arm
No ethernet found.
missing environment variable : bootfile
Retrieving fie: pxelinux.cfg/default
No ethernet found.
Config file not found.
No ethernet found.
No ethernet found.
U-boot>

How can Fix this?

Looks like the included u-boot not support boot from usb because it not set matching environment variables for boot_dev. Our skripts set this to mmc if it was empty.