[clock] sync os time to CMOS before rebooting (sonic-net#465)

So if CMOS clock has drifted, we fix it at reboot time after syncing
with NTP.

Signed-off-by: Ying Xie <ying.xie@microsoft.com>

scripts/fast-reboot

@@ -360,6 +360,11 @@ sync
 sleep 1
 sync
 
+# sync the current system time to CMOS
+if [ -x /sbin/hwclock ]; then
+    /sbin/hwclock -w || /bin/true
+fi
+
 # Reboot: explicity call Linux native reboot under sbin
 debug "Rebooting with ${REBOOT_METHOD} to ${NEXT_SONIC_IMAGE} ..."
 exec ${REBOOT_METHOD}

scripts/reboot

@@ -48,6 +48,11 @@ echo "User issued 'reboot' command [User: ${REBOOT_USER}, Time: ${REBOOT_TIME}]"
 sync
 sleep 3
 
+# sync the current system time to CMOS
+if [ -x /sbin/hwclock ]; then
+    /sbin/hwclock -w || /bin/true
+fi
+
 if [ -x ${DEVPATH}/${PLATFORM}/${PLAT_REBOOT} ]; then
     echo "Rebooting with platform ${PLATFORM} specific tool ..."
     exec ${DEVPATH}/${PLATFORM}/${PLAT_REBOOT} $@