Banana Pi R1 sd card not detected

Hello,
I'm using OpenWRT on quite some devices, recently I got my hands on a Banana Pi R1 lambo board. I'd like to use it as VLAN capable switch and Wifi router. So the lack of separation between the WAN and LAN ports is no issue for me.

So much for the general introduction let's get to the issue, I tried various OpenWRT images but none of them did seem to boot (except of the extremely outdated 15.05 but even there network didn't really work.

After trying latest 24 version I finaly got a clue, as the latest uboot does enable HDMI I can at least see it's boot messages and found out that while uboot is loaded from SD card it failes to detect the SD card afterwards
"No MMC detected"

Seems like the rest is working fine as uboot does get a ip address from dhcp and tries to boot from network.
I haven't tired booting the kernel from network by now but that'd be an experiment only anyway.

Does anyone have an idea why uboot believes no sdcard is plugged?

Thanks.