vpl: Allow signature verification

Add the required Kconfig option so that signatures can be verified when
loading a configuration.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/Kconfig b/common/Kconfig
index 05c5e1c..21434c5 100644
--- a/common/Kconfig
+++ b/common/Kconfig
@@ -1065,6 +1065,13 @@
 	help
 	  Enable image_sign_info helper functions in SPL.
 
+config VPL_IMAGE_SIGN_INFO
+	bool
+	select SHA1
+	select SHA256
+	help
+	  Enable image_sign_info helper functions in SPL.
+
 endif
 
 config FDT_SIMPLEFB