Fix image creation without sign/encryption

Generating images with no signature or encryption was broken by commit
06b77b835336d80c102e35f7c6da997d9380c9b8

This allows generating images with just sha256 again, and fixes a few
leftovers from the imghash TLV change.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/scripts/imgtool/image.py b/scripts/imgtool/image.py
index 8087447..6116420 100644
--- a/scripts/imgtool/image.py
+++ b/scripts/imgtool/image.py
@@ -148,7 +148,7 @@
                 raise Exception(msg)
 
     def create(self, key, enckey):
-        self.add_header(key, enckey)
+        self.add_header(enckey)
 
         tlv = TLV(self.endian)
 
@@ -189,11 +189,8 @@
 
         self.payload += tlv.get()
 
-    def add_header(self, key, enckey):
-        """Install the image header.
-
-        The key is needed to know the type of signature, and
-        approximate the size of the signature."""
+    def add_header(self, enckey):
+        """Install the image header."""
 
         flags = 0
         if enckey is not None: