nixos/timesyncd: initialize clock file with current time

When initializing a system (e.g. first boot / livecd) we have no good
reference source for time. systemd-timesyncd however would revert back
to its configured fallback time (in our case 01.01.1980). Since we
probably don't want to hardcode a specific date as fallback we are now
using the current system time (wherever that might have come from) to
initialize the reference clock file.

The only systems that might be remotely affected by this change are
machines that have highly unreliable RTCs or those where the battery
that backs the RTC is running empty.

Historically these systems always had a tough time with anything time
related and likely required manual intervention.

For stateless systems (those that wipe / between reboots or our
installer CDs) this has the consequence that time will always be reset
to whatever the system comes up with on boot. This is likely the correct
time coming from an RTC. No harm done here the situation is likely
unchanged for them.

For stateful systems (those that retain the / partition across reboots)
there shouldn't be a change at all. They'll provide an initial clock
value once on their lifetime (during first boot / after installation).
From then onwards systemd-timesyncd will update the file with the newer
fallback time (that will be picked up on the next boot).

nixos/modules/system/boot/timesyncd.nix

@@ -60,15 +60,27 @@ with lib;
     };
     users.groups.systemd-timesync.gid = config.ids.gids.systemd-timesync;
 
-    system.activationScripts.systemd-timesyncd-migration = mkIf (versionOlder config.system.stateVersion "19.09") ''
+    system.activationScripts.systemd-timesyncd-migration =
       # workaround an issue of systemd-timesyncd not starting due to upstream systemd reverting their dynamic users changes
       #  - https://github.com/NixOS/nixpkgs/pull/61321#issuecomment-492423742
       #  - https://github.com/systemd/systemd/issues/12131
-      if [ -L /var/lib/systemd/timesync ]; then
-        rm /var/lib/systemd/timesync
-        mv /var/lib/private/systemd/timesync /var/lib/systemd/timesync
+      mkIf (versionOlder config.system.stateVersion "19.09") ''
+        if [ -L /var/lib/systemd/timesync ]; then
+          rm /var/lib/systemd/timesync
+          mv /var/lib/private/systemd/timesync /var/lib/systemd/timesync
+        fi
+      '';
+    system.activationScripts.systemd-timesyncd-init-clock =
+      # Ensure that we have some stored time to prevent systemd-timesyncd to
+      # resort back to the fallback time.
+      # If the file doesn't exist we assume that our current system clock is
+      # good enough to provide an initial value.
+      ''
+      if ! [ -f /var/lib/systemd/timesync/clock ]; then
+        test -d /var/lib/systemd/timesync || mkdir -p /var/lib/systemd/timesync
+        touch /var/lib/systemd/timesync/clock
       fi
-    '';
+      '';
   };
 
 }