No rpi3 image currently works on rpi3-model B+

On my Raspberry Pi 3, model B+, neither the current testing nor unstable images work (April 22, 2018).

The testing image (http://ftp.freedombox.org/pub/freedombox/testing-latest/freedombox-testing-nonfree_latest_raspberry3-armhf.img.xz) doesn't seem to boot at all. No blinking of the LEDs (just "power on"), no screen output over HDMI.

The unstable image (https://ftp.freedombox.org/pub/freedombox/nightly/freedombox-unstable-nonfree_dailydebian_raspberry3-armhf.img.xz) seems slightly more promising. Note I didn't connect any ethernet port. I transcribed the screen output:

Net: No ethernet found.
starting USB...
USB0: Core Release: 2.80a
scanning bus for devices... 4 USB Device(s) found
     scanning usb for storage devices... 0 Storage Device(s) found
Hit any key to stop autoboot:  0
switch to partitions #0, OK
mmc0 is current device
Scanning mmc 0:2...
Found U-Boot script /boot.scr
2904 bytes read in 8 ms (354.5 KiB/s)
## Exectuing script at 02000000
4043264 bytes read in 167 ms (23.1 MiB/s)
** File not found /dtbs/4.15.0-2-armmp/bcm283x-rpi-other.dtb **
SCRIPT FAILED: continuing...

Device 0: unknown device
No ethernet found.
missing environment variable: pxeuuid
missing environment variable: bootfile
Retrieving file: pxelinux.cfg/00000000
No ethernet found.
missing environment variable: bootfile
Retrieving file: pxelinux.cfg/0000000
No ethernet found.
missing environment variable: bootfile
Retrieving file: pxelinux.cfg/000000
No ethernet found.
missing environment variable: bootfile
Retrieving file: pxelinux.cfg/00000
No ethernet found.
missing environment variable: bootfile
Retrieving file: pxelinux.cfg/0000
No ethernet found.
missing environment variable: bootfile
Retrieving file: pxelinux.cfg/000
No ethernet found.
missing environment variable: bootfile
Retrieving file: pxelinux.cfg/00
No ethernet found.
missing environment variable: bootfile
Retrieving file: pxelinux.cfg/0
No ethernet found.
missing environment variable: bootfile
Retrieving file: pxelinux.cfg/default-arm-bcm283x
No ethernet found.
missing environment variable: bootfile
Retrieving file: pxelinux.cfg/default-arm
No ethernet found.
missing environment variable: bootfile
Retrieving file: pxelinux.cfg/default
No ethernet found.
Config file not found
No ethernet found.
No ethernet found.
U-Boot>

I'm happy to get more information, if you tell me how :).