OK - Found the line in http://linux-sunxi.org/LCD:
#5" LVDS LCD
CONFIG_VIDEO_LCD_MODE="x:800,y:480,depth:24,pclk_khz:30000,le:40,ri:40,up:29,lo:13,hs:48,vs:3,sync:3,vmode:0"
CONFIG_VIDEO_LCD_POWER="PH12"
CONFIG_VIDEO_LCD_BL_EN="PH8"
CONFIG_VIDEO_LCD_BL_PWM="PB2"
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?
Please, give me a hint!
BR
Guido