[mellanox] Fix in mlnx-ffb.sh (#2676)

Fixes "No ISSU version file found /etc/mlnx/issu-version"
when rebooting to different image;
Add aditional check condition.

Signed-off-by: Stepan Blyschak <stepanb@mellanox.com>

platform/mellanox/mlnx-ffb.sh

@@ -42,11 +42,16 @@ check_sdk_upgrade()
 
         ISSU_VERSION_FILE_PATH="/etc/mlnx/issu-version"
 
-        [ -f "${SDK_VERSION_FILE_PATH}" ] || {
+        [ -f "${ISSU_VERSION_FILE_PATH}" ] || {
             >&2 echo "No ISSU version file found ${ISSU_VERSION_FILE_PATH}"
             break
         }
 
+        [ -f "${FS_MOUNTPOINT}/${ISSU_VERSION_FILE_PATH}" ] || {
+            >&2 echo "No ISSU version file found ${ISSU_VERSION_FILE_PATH} in ${NEXT_SONIC_IMAGE}"
+            break
+        }
+
         CURRENT_ISSU_VERSION="$(cat ${ISSU_VERSION_FILE_PATH})"
         NEXT_ISSU_VERSION="$(cat ${FS_MOUNTPOINT}/${ISSU_VERSION_FILE_PATH})"