tools: imx9_image: Reword warning message.

If the optional `tee.bin` OP-TEE binary is missing, the image will still be
functional. Adapt the warning message accordingly.

Signed-off-by: Mathieu Othacehe <othacehe@gnu.org>
diff --git a/tools/imx9_image.sh b/tools/imx9_image.sh
index 88dfcfe..ca78a57 100755
--- a/tools/imx9_image.sh
+++ b/tools/imx9_image.sh
@@ -19,7 +19,7 @@
 	fi
 
 	if [ ! -f $tmp ]; then
-		echo "WARNING '$tmp' not found, resulting binary is not-functional" >&2
+		echo "WARNING '$tmp' not found, resulting binary may be not-functional" >&2
 
                 # Comment-out the lines for un-existing files. This way,
                 # mkimage can keep working. This allows CI tests to pass even