Hello,
I have the same problem with a BananaPI and Debian buster.
u-boot from buster does not accept any key press and autoboot starts the installer on a serial console (blank screen).
I have tried a "trick" and created a SD-card with firmware.BananaPi.img.gz from stretch and partition.img.gz from buster. Now I can enter setenv commands on the u-boot console and start the installer. But then I have no keyboard in the installer.
Markus
https://get.debian.org/debian/dists/stretch/main/installer-armhf/current/images/hd-media/SD-card-images/firmware.BananaPi.img.gz
https://get.debian.org/debian/dists/testing/main/installer-armhf/20190702/images/hd-media/SD-card-images/partition.img.gz
