systemd: backport serialization change from v234

Bisecting between systemd v232 and v234 to find the source of
the improvement in behavior of the serial tty lead to commit
e266c068b5597e18b2299f9c9d3ee6cf04198c41.  Backport the change
to hopefully fix the failures.

Bug-AGL: SPEC-544

Change-Id: I7b3d18c6603d0163ab66aaf94daac6fead2e6d3d
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/11139
Reviewed-by: Matt Ranostay <matt.ranostay@konsulko.com>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>