system-update: Reload systemd before beginning

This will silence warnings from `mount` about `/etc/fstab` having
changed since the last time the `systemd-fstab-generator` was run.  It's
not true, since it's immutable, but the warning comes up on Raspberry
Pis without an RTC.

overlay/usr/bin/system-update

@@ -119,6 +119,8 @@ trap cleanup INT TERM QUIT EXIT
 workdir=$(mktemp -d)
 cd "${workdir}"
 
+systemctl daemon-reload
+
 fetch_update "${source_url}" || die 'Failed to fetch update source'
 extract_update || die 'Failed to extact update source'
 ./install "${newroot}" || die 'Error installing system update'