Add bootutil support for RSA-3072

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/bootutil/src/image_rsa.c b/boot/bootutil/src/image_rsa.c
index 0534c12..53a4591 100644
--- a/boot/bootutil/src/image_rsa.c
+++ b/boot/bootutil/src/image_rsa.c
@@ -39,7 +39,7 @@
  */
 
 /* The size, in octets, of the message. */
-#define PSS_EMLEN 256
+#define PSS_EMLEN (MCUBOOT_SIGN_RSA_LEN / 8)
 
 /* The size of the hash function.  For SHA256, this is 32 bytes. */
 #define PSS_HLEN 32
@@ -48,7 +48,7 @@
 #define PSS_SLEN 32
 
 /* The length of the mask: emLen - hLen - 1. */
-#define PSS_MASK_LEN (256 - PSS_HLEN - 1)
+#define PSS_MASK_LEN (PSS_EMLEN - PSS_HLEN - 1)
 
 #define PSS_HASH_OFFSET PSS_MASK_LEN