After a number of ties to boot the old system using the "bootm" command, it happens that your kernel (version) doesn't fit it's own root the file system (version). At the end of the boot process it stops with the error "kernel too old".
However I found a fully equipped Ubuntu 14.04 LTS (kernel version 3.10.17) here --> https://blog.janprunk.com/?p=866 where both ethernet sockets (eth0, eth1) are working. :-)
Thus, there must be something wrong with the kernel configuration and/or compilation process of the newer kernels...