Remove exec from platform_reboot_plugin call to handle any hang issue. (

sonic-net#1879)

What I did
Remove exec from the platform_reboot call to handle any hang issue during reboot

How I did it
Remove "exec" from " exec ${DEVPATH}/${PLATFORM}/${PLAT_REBOOT} $@"

How to verify it
Perform sudo reboot after DE initializing the platfom driver and see if the device gracefully reboots by /sbin/reboot after failing on platform_reboot.

scripts/reboot

@@ -198,7 +198,7 @@ fi
 
 if [ -x ${DEVPATH}/${PLATFORM}/${PLAT_REBOOT} ]; then
     VERBOSE=yes debug "Rebooting with platform ${PLATFORM} specific tool ..."
-    exec ${DEVPATH}/${PLATFORM}/${PLAT_REBOOT} $@
+    ${DEVPATH}/${PLATFORM}/${PLAT_REBOOT} $@
 
     # There are a couple reasons execution reaches here:
     #