To activate debug level, add to units under [Service] this line
Environment=SYSTEMD_LOG_LEVEL=debug

Note on systemd-versions: Some systemds below 204 (verified in
systemd-195) seem to need libpci-dev (or the openSuse equivalent).


Another note on our compiled systemd-udevd:
So far it has not been possible to reach a fully functional udev
system in view of the perceived systemd-udevd limitation about 
evaluating the variable ENV{DEVTYPE}=="usb_device" in udev rule
scripts.