@Kwiboo did a fantastic job. Mainline u-boot becomes usable (console.log) for me.
It enumerates my different systems on different storage media and can natively load the boot components from them.
The use of compressed kernels does not work yet, it bails out like this:
The u-boot control still has to be done with the serial console, as HDMI support is still missing, but it is still an early stage and the development is vivid.
In the meantime, I have solved the problem with loading compressed kernels for me. Also, I can now use a USB keyboard (usbkbd) for stdin. Both require a correction of the mainline code, but since unfortunately too many in-flight patches have not yet landed, it is still too early to go into detail here. If you already want to play with the functionality that will be provided by the mainline u-boot in the future out-of-the-box, I have uploaded my firmware build here. The firmware can be put in place by:
dd bs=512 seek=64 conv=notrunc,fsync if=u-boot-rockchip.bin of=/dev/${entire-device-to-be-used}
into the firmware area of an SD card or an eMMC. If it is started during power-up by holding the SPI recovery button (RCY), it will scan all connected storage media for a usable bootflow.
Supported boot methodes are: extlinux (extlinux.conf), efiboot and legacy-boot (boot.scr).
As soon as video support is available, "U-Boot Standard Boot" will be fully usable, but VOB2 support is still open.