systemd-shutdown: adjust watchdog expiration time

By default, the second stage watchdog time that is used to set the
watchdog once systemd-shutdown has stopped all services and replaced
PID1 with itself is set to 10m.

However, the RPI watchdog has a maximum timeout of 15s, so we adjust this
value accordingly.

This helps ensure that when a shutdown or reboot is triggered, the device
can fallback to a watchdog reboot if it gets stuck.

Changelog-entry: adjust reboot watchdog timeout
Signed-off-by: Alex Gonzalez <alexg@balena.io>

layers/meta-balena-raspberrypi/recipes-core/systemd/systemd/rpi/watchdog.conf

@@ -0,0 +1,3 @@
+[Manager]
+RuntimeWatchdogSec=10s
+RebootWatchdogSec=15s