Alright, I thought there was some other location than the fstab for these "system" partitions with my setup. Now I tried to enter the root partition with my additional options in fstab, which did work in fact. I do have some problems with the file system checks (fsck) at boot now that I didn't have before (even with setting the fstab pass flag to to 0), but I'll also get this sorted out somehow.

Thanks for the help!

A related question:

I wanted to add some mount options to the root filesystem, like noatime to reduce the amount of write operations to the root fs (installed on the SD card).

However, I don't quite know where to do this, now that there are no entries in fstab. Can someone give me an advice of where to define sth. like this instead?

Thanks already.