Enable root autologin on serial console

See QubesOS/qubes-issues#5799.

Use an option to agetty:
  https://wiki.archlinux.org/index.php/Getty#Automatic_login_to_virtual_console

The --login-pause causes agetty to wait for Enter key. This is
important, because otherwise the root session prevents systemd from
shutting down, and probably causes other side effect.

Makefile

@@ -54,7 +54,7 @@ USER_DROPIN_DIR ?= "usr/lib/systemd/user"
 
 SYSTEM_DROPINS := boot.automount chronyd.service crond.service
 SYSTEM_DROPINS += cups.service cups-browsed.service cups.path cups.socket ModemManager.service
-SYSTEM_DROPINS += getty@tty.service
+SYSTEM_DROPINS += getty@tty.service serial-getty@.service
 SYSTEM_DROPINS += tmp.mount
 SYSTEM_DROPINS += org.cups.cupsd.service org.cups.cupsd.path org.cups.cupsd.socket
 SYSTEM_DROPINS += systemd-random-seed.service

debian/qubes-core-agent.install

@@ -88,6 +88,7 @@ lib/systemd/system/qubes-updates-proxy-forwarder@.service
 lib/systemd/system/qubes-updates-proxy-forwarder.socket
 lib/systemd/system/qubes-sync-time.service
 lib/systemd/system/qubes-sync-time.timer
+lib/systemd/system/serial-getty@.service.d/30_qubes.conf
 lib/systemd/system/systemd-random-seed.service.d/30_qubes.conf
 lib/systemd/system/tinyproxy.service.d/30_not_needed_in_qubes_by_default.conf
 lib/systemd/system/tmp.mount.d/30_qubes.conf

rpm_spec/core-agent.spec.in

@@ -869,6 +869,7 @@ The Qubes core startup configuration for SystemD init.
 /lib/systemd/system/ModemManager.service.d/30_qubes.conf
 /lib/systemd/system/NetworkManager.service.d/30_qubes.conf
 /lib/systemd/system/NetworkManager-wait-online.service.d/30_qubes.conf
+/lib/systemd/system/serial-getty@.service.d/30_qubes.conf
 /lib/systemd/system/systemd-random-seed.service.d/30_qubes.conf
 /lib/systemd/system/systemd-timesyncd.service.d/30_qubes.conf
 /lib/systemd/system/tinyproxy.service.d/30_not_needed_in_qubes_by_default.conf

vm-systemd/serial-getty@.service.d/30_qubes.conf

@@ -0,0 +1,3 @@
+[Service]
+ExecStart=
+ExecStart=-/sbin/agetty --autologin root --login-pause --keep-baud 115200,38400,9600 %I $TERM