warp7_bl33: configs: Enable CONFIG_OF_LIBFDT_OVERLAY

This commit enables CONFIG_OF_LIBFDT_OVERLAY a requirement to perform a
merge of an OPTEE provided DTB overlay into our main kernel DTB image.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
diff --git a/configs/warp7_bl33_defconfig b/configs/warp7_bl33_defconfig
index d2cf84b..0f28d58 100644
--- a/configs/warp7_bl33_defconfig
+++ b/configs/warp7_bl33_defconfig
@@ -61,3 +61,4 @@
 CONFIG_FIT_SIGNATURE=y
 CONFIG_FIT_VERBOSE=y
 CONFIG_OF_LIBFDT=y
+CONFIG_OF_LIBFDT_OVERLAY=y