image: Add signing infrastructure

Add a structure to describe an algorithm which can sign and (later) verify
images.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/README b/README
index 7196fde..d3e03d7 100644
--- a/README
+++ b/README
@@ -2805,6 +2805,11 @@
 		most specific compatibility entry of U-Boot's fdt's root node.
 		The order of entries in the configuration's fdt is ignored.
 
+		CONFIG_FIT_SIGNATURE
+		This option enables signature verification of FIT uImages,
+		using a hash signed and verified using RSA. See
+		doc/uImage.FIT/signature.txt for more details.
+
 - Standalone program support:
 		CONFIG_STANDALONE_LOAD_ADDR