Short update:
The SD card with letux-4.19.87 was incomplete because the kernel is missing on the download server.
If letux-4.19.88 .. 91 is installed on an SD card containing letux-4.19.86 or prior (without wiping the filesystem) the system works too.
The reason might be hidden in the modules.tgz archive. Archives newer than letux-4.19.86 containing only the bare lib folder. Older archives also carried content for folder etc, root and usr.
The SD card with letux-4.19.87 was incomplete because the kernel is missing on the download server.
If letux-4.19.88 .. 91 is installed on an SD card containing letux-4.19.86 or prior (without wiping the filesystem) the system works too.
The reason might be hidden in the modules.tgz archive. Archives newer than letux-4.19.86 containing only the bare lib folder. Older archives also carried content for folder etc, root and usr.
- download the file device-trees.tbz, modules.tgz and uImage for the kernel version you like to install (eg: http://download.goldelico.com/letux-kernel/letux-4.19.91/)
- mount you SD card and change into the root folder of the SD card (eg. /media/${USER}/rootfs)
- extract the modules archive into the current folder
- change into the boot folder of the SD card
- copy the uImage file into the current folder
- extract the device-trees.tbz into the current folder