work around annoying mount messages

Recent versions of "mount" print a warning message when /etc/fstab is newer
than /run/systemd/systemd-units-load.
Since the latter is created during early boot when the system time
hasn't been set yet, the warning is printed pretty much always, at least
on Raspberry Pi which doesn't have an RTC.
Suppress the message by setting the modification date of /etc/fstab to
Jan 1 1970, so that it always appears older than systemd-units-load.

setup/pi/make-root-fs-readonly.sh

@@ -189,4 +189,8 @@ then
   echo "tmpfs /var/lib/ntp tmpfs nodev,nosuid 0 0" >> /etc/fstab
 fi
 
+# work around 'mount' warning that's printed when /etc/fstab is
+# newer than /run/systemd/systemd-units-load
+touch -t 197001010000 /etc/fstab
+
 log_progress "done"