systemd unit: use StandardInput=null to work in serial console mode

systemd unit: use StandardInput=null to work in serial console mode

We shouldn't need standard input connected to a TTY, but when booting
in serial console mode (like `console=tty1 console=ttyS0,9600n8`), then
grml-autoconfig service fails with:

| systemd[1]: Starting Grml boot option support...
| systemd[1]: grml-autoconfig.service: Main process exited, code=exited, status=1/FAILURE
| systemd[1]: grml-autoconfig.service: Failed with result 'exit-code'.
| systemd[1]: Failed to start Grml boot option support.
| systemd[1]: grml-autoconfig.service: Consumed 2.475s CPU time.

Replacing `StandardInput=tty` with `StandardInput=null` fixes
this, while non-serial console mode seems to continue working fine.

Closes: https://github.com/grml/grml/issues/176