Add bootutil support for RSA-3072

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/docs/design.md b/docs/design.md
index 13f60b1..5c3d1b9 100644
--- a/docs/design.md
+++ b/docs/design.md
@@ -96,6 +96,7 @@
 #define IMAGE_TLV_RSA2048_PSS       0x20   /* RSA2048 of hash output */
 #define IMAGE_TLV_ECDSA224          0x21   /* ECDSA of hash output */
 #define IMAGE_TLV_ECDSA256          0x22   /* ECDSA of hash output */
+#define IMAGE_TLV_RSA3072_PSS       0x23   /* RSA3072 of hash output */
 ```
 
 Optional type-length-value records (TLVs) containing image metadata are placed