Until a lot of search I found the reason, why the initramfs don't work for me.
I have try to add a initramfs.cpio.gz archive to the kernel. In the kernel I have configured to compress the archive with lzma (and not with gz). So I have also only activate the lzma uncompress support.
The problem was, that the kernel makefile do not unpack and repack the initramfs with the selected compression system, like I understand the kernel documentation. Because of this the kernel was not able to unpack the included initramfs.
A second problem was, that the RPI in this case do not shows the kernel boot messages, like it done if no initramfs is included, but shows only the debug color screen. So we thought that the RPI was not able to load the kernel.
Today we have a running kernel and also a running MLD Distribution http://www.minidvblinux.de/site/cont_in ... sub_id=918
. OK, there is some fine tuning necessary, but its already running