[reboot] Add platform-specific reboot cause update hook (sonic-net#1454)

#### What I did
The S6000 devices, the cold reboot is abrupt and it is likely to cause issues which will cause the device to land into EFI shell. Hence the platform reboot will happen after graceful unmount of all the filesystems as in S6100.

#### How I did it
In reboot script, if platform-specific reboot cause update script exists, run it

scripts/reboot

@@ -1,6 +1,7 @@
 #!/bin/bash
 DEVPATH="/usr/share/sonic/device"
 PLAT_REBOOT="platform_reboot"
+PLATFORM_UPDATE_REBOOT_CAUSE="platform_update_reboot_cause"
 REBOOT_CAUSE_FILE="/host/reboot-cause/reboot-cause.txt"
 REBOOT_TIME=$(date)
 
@@ -167,6 +168,11 @@ if [ -x ${DEVPATH}/${PLATFORM}/${SSD_FW_UPDATE} ]; then
     ${DEVPATH}/${PLATFORM}/${SSD_FW_UPDATE} ${REBOOT_TYPE}
 fi
 
+if [ -x ${DEVPATH}/${PLATFORM}/${PLATFORM_UPDATE_REBOOT_CAUSE} ]; then
+    debug "updating reboot cause for ${PLATFORM}"
+    ${DEVPATH}/${PLATFORM}/${PLATFORM_UPDATE_REBOOT_CAUSE}
+fi
+
 if [ -x ${DEVPATH}/${PLATFORM}/${PLAT_REBOOT} ]; then
     VERBOSE=yes debug "Rebooting with platform ${PLATFORM} specific tool ..."
     exec ${DEVPATH}/${PLATFORM}/${PLAT_REBOOT} $@