systemd: Sync systemd-serialgetty@.service with upstream

The systemd upstream has been doing some improvements in the service
which were not in sync, with the forked file. Mainly:

 - Use BindsTo to bind the service with the required serial device
 - Add of getty-pre.target as dependency so we can run things before
   getty@.service and serial-getty@.service
 - Add conflicts to rescue to avoid issues with sulogin

We did not change the ExecStart and Environment fields so we don't have
side effects of this change.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>