Thank you everybody for all your answers!
Might be worth taking a look at Dracut, too. It isn't in ports but I think is still simpler than rolling your own initramfs the old way.
I could probably use kernel/linux-3.xx.y/user/gen_init_cpio, but I think I'll have a look at the Dracut tool. I think it will be easier. Thank your for providing the Pkgfile! I will boot with the iso, set a network connection and build the tool with your Pkgfile. I hope it will works.
Device Boot Start End Sectors Size Id Type /dev/sdd1 2048 10487807 10485760 5G 83 Linux /dev/sdd2 10487808 976773167 966285360 460.8G fd Linux raid autodetect
and then the md-superblock knows the rest (i.e., which name the raid has, which type, etc.).
... there's also "8e Linux LVM", so maybe this works, too.
I have used "Linux LVM" and it's not enough. Thank you for your link Cezar. I don't remember if I saw that link. As it is said it's not possible without an initrd, but it could work with grub2. As I want to use syslinux, I didn't keep that option. But I will read it carefully one more time to understand the problem better. To answer Erich in the past I didn't used lvm. It happened that I had to reinstall everything, because I had some problem with my partitions size. At the end I decided I don't want anymore to reinstall everything because a partition is too small. So lvm was the solution. Best Regards, Cédric