doc: design: fix image flags

Copy up to date image flags from image.h. Fixes for:

- Wrong comment for IMAGE_F_ECDSA224_SHA256
- Missing definition for IMAGE_F_PKCS1_PSS_RSA2048_SHA256

Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>
diff --git a/doc/design.txt b/doc/design.txt
index 384fc5e..bc4abea 100644
--- a/doc/design.txt
+++ b/doc/design.txt
@@ -79,12 +79,13 @@
 /*
  * Image header flags.
  */
-#define IMAGE_F_PIC                   0x00000001 /* Not currently supported. */
-#define IMAGE_F_SHA256                0x00000002 /* Image contains hash TLV */
-#define IMAGE_F_PKCS15_RSA2048_SHA256 0x00000004 /* PKCS15 w/RSA and SHA */
-#define IMAGE_F_ECDSA224_SHA256       0x00000008 /* ECDSA256 over SHA256 */
-#define IMAGE_F_NON_BOOTABLE          0x00000010 /* Split image app. */
-#define IMAGE_F_ECDSA256_SHA256       0x00000020 /* ECDSA256 over SHA256 */
+#define IMAGE_F_PIC                      0x00000001 /* Not supported. */
+#define IMAGE_F_SHA256                   0x00000002 /* Hash TLV is present */
+#define IMAGE_F_PKCS15_RSA2048_SHA256    0x00000004 /* PKCS15 w/RSA and SHA */
+#define IMAGE_F_ECDSA224_SHA256          0x00000008 /* ECDSA224 over SHA256 */
+#define IMAGE_F_NON_BOOTABLE             0x00000010 /* Split image app. */
+#define IMAGE_F_ECDSA256_SHA256          0x00000020 /* ECDSA256 over SHA256 */
+#define IMAGE_F_PKCS1_PSS_RSA2048_SHA256 0x00000040 /* PKCS1 PSS */
 
 /*
  * Image trailer TLV types.