Make sure to include LZMA support into the kernel (patch is on udpcast site)

Right, that was the problem.. was fooled by  "file" which didn't know that format, sorry !

Now,  I've recompiled a kernel with the necessary options to get a serial console, but I loose output after udpc_dialog starts. So I'm trying to modify the inittab in initrd to start ash instead and see what's wrong.  Unfortunately I'm failing to build an initrd the kernel will recognize (unpacked the original, modified it, cpio, lzma e ...)
Seems I need initramfs-tools but in turns this requires a newer udev on debian stable....

Any easier way ?  I've tried a cramFS image with no luck.