I just saw that the missing libkmod (dlopen'ed by udevadm) is already taken care of by /usr/share/initramfs-tools/hooks/udev.
In case you're wondering what caused libsystemd.so.0 to be included in my initrd-img: it's linked by /usr/sbin/lvm which gets added by /usr/share/initramfs-tools/hooks/lvm2 Cheers, Roderich