[201911] platform pre-check for reboot (#1472)

What I did
Added platform pre check support in reboot script.
Checking platform based changes before stopping dockers and sonic services.
How I did it
On branch reboot_pre_check_201911
Changes not staged for commit:
(use "git add ..." to update what will be committed)
(use "git checkout -- ..." to discard changes in working directory)

    modified:   scripts/reboot
How to verify it
Write a platform pre check script(platform_reboot_pre_check) and place it in /usr/share/sonic/device/<HWSKU>/ directory.
If the script exit with status 0, reboot will be proceeded.
If script exit with non-zero status, the reboot script gets stopped.

scripts/reboot

@@ -7,6 +7,7 @@ ASIC_TYPE=$(sonic-cfggen -y /etc/sonic/sonic_version.yml -v asic_type)
 DEVPATH="/usr/share/sonic/device"
 PLAT_REBOOT="platform_reboot"
 REBOOT_CAUSE_FILE="/host/reboot-cause/reboot-cause.txt"
+PLATFORM_REBOOT_PRE_CHECK="platform_reboot_pre_check"
 VERBOSE=no
 EXIT_NEXT_IMAGE_NOT_EXISTS=4
 
@@ -72,6 +73,11 @@ function reboot_pre_check()
     fi
     rm ${filename}
 
+    if [ -x ${DEVPATH}/${PLATFORM}/${PLATFORM_REBOOT_PRE_CHECK} ]; then
+        ${DEVPATH}/${PLATFORM}/${PLATFORM_REBOOT_PRE_CHECK}
+        [[ $? -ne 0 ]] && exit
+    fi
+
     # Make sure that the next image exists
     if [[ ! -d ${IMAGE_PATH} ]]; then
         VERBOSE=yes debug "Next image ${NEXT_SONIC_IMAGE} doesn't exist ..."