[202012][Mellanox]: Update ONiE FW tool: manual reboot control. (#13359)

Partial cherry-pick of: [Mellanox] Modified Platform API to support all firmware updates in single boot #9608

- Why I did it
To allow user manual reboot control over ONiE FW upgrade

- How I did it
Added a dedicated script argument handling

- How to verify it
mlnx-onie-fw-update.sh update --no-reboot

Signed-off-by: Nazarii Hnydyn <nazariig@nvidia.com>

platform/mellanox/mlnx-onie-fw-update.sh

@@ -174,6 +174,10 @@ case "${cmd}" in
             rc=$?
             disable_onie_access
             if [[ ${rc} -eq 0 ]]; then
+                if [[ "${arg}" == "--no-reboot" ]]; then
+                    echo "INFO: ONIE firmware update successfully STAGED for install at NEXT reboot. Please reboot manually to complete installation"
+                    exit 0
+                fi
                 system_reboot
             else
                 echo "ERROR: failed to enable ONIE firmware update mode"