OK - Found the line in http://linux-sunxi.org/LCD:
#5" LVDS LCD
Adding this in /home/<USER>/build/cache/sources/u-boot/v*/configs/Bananapro_defconfig
and adding the pwm-section (exactly as above) in /home/<user>/build/cache/sources/u-boot/v*/arch/arm/dts/sun7i-a20-bananapro.dts and /home/<USER>/build/cache/sources/linux-mainline/orange-pi-5.4/arch/arm/boot/dts/sun7i-a20-bananapro.dts at the given time while executing ./compile.sh CREATE_PATCHES=yes creates the deb-files,
from which I applied the named three on a fresh installation from an image made by ./compile.sh without modifications.
But still - no output on Display - what have I overseen?
PWM is working and I can controll it via gpio34 as described above.
In the description is written, that 7"-Display is LVMS and 5"-Display is RGB, what doesn't make much sense, but I think this is meant by the presence ofthe Line CONFIG_VIDEO_LCD_PANEL_LVDS=y in case of 7"-LCD and the absence in case of 5"-LCD.
Is there anything else to be done? Any special driver to be activated, which isn't active by default?
I tested the actual Xenial on my BPP but got a crashdump each time I reboot or shutdown the system.
I found, that armhwinfo was causing this crash by removing the module a20_tp.
Disabling the line
/sbin/modprobe -r $(cut -f1 -d' ' </proc/modules)
in /etc/init.d/armhwinfo avoid this problem.
This line is part of a patch Thomas Kaiser introduced in October last year.
I don't know, if this problem occurs only in my configuration (nothing special, but a connected Lemaker 5" LCD may be the difference), or it is related to the Kernel 3.4.113 only, but it goes reproducably wrong, if this line is active. It cause the oposite of the intention of this patch, because after the crash, the next module (sunxi_ir) is ending in a loop. So no sync is executed and the power stays on.
Without this line, all works well.
I think, the patch has to be patched again - don't touch a20_tp.