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.

(From OE-Core rev: 63bbff61b78c651339c4b18d8376187379ec3b3c)

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

meta/recipes-core/systemd/systemd-serialgetty/serial-getty@.service

@@ -1,3 +1,5 @@
+#  SPDX-License-Identifier: LGPL-2.1+
+#
 #  This file is part of systemd.
 #
 #  systemd is free software; you can redistribute it and/or modify it
@@ -9,9 +11,8 @@
 Description=Serial Getty on %I
 Documentation=man:agetty(8) man:systemd-getty-generator(8)
 Documentation=http://0pointer.de/blog/projects/serial-console.html
-PartOf=dev-%i.device
-ConditionPathExists=/dev/%i
-After=dev-%i.device systemd-user-sessions.service plymouth-quit-wait.service
+BindsTo=dev-%i.device
+After=dev-%i.device systemd-user-sessions.service plymouth-quit-wait.service getty-pre.target
 After=rc-local.service
 
 # If additional gettys are spawned during boot then we should make
@@ -20,12 +21,17 @@ After=rc-local.service
 Before=getty.target
 IgnoreOnIsolate=yes
 
+# IgnoreOnIsolate causes issues with sulogin, if someone isolates
+# rescue.target or starts rescue.service from multi-user.target or
+# graphical.target.
+Conflicts=rescue.service
+Before=rescue.service
+
 [Service]
 Environment="TERM=xterm"
 ExecStart=-/sbin/agetty -8 -L %I @BAUDRATE@ $TERM
 Type=idle
 Restart=always
-RestartSec=0
 UtmpIdentifier=%I
 TTYPath=/dev/%I
 TTYReset=yes