imgtool: fix encrypting hex images

Fixes padding hex images when encrypting. The issues stems from binaries
using `bytes` and IntelHex returning `array` where `bytes` cannot be
appended to, so use `.extend()` instead.

Signed-off-by: Fabio Utzig <utzig@apache.org>
Signed-off-by: David Brown <david.brown@linaro.org>

Author: utzig

scripts/imgtool/image.py

@@ -352,7 +352,11 @@ def create(self, key, public_key_format, enckey, dependencies=None,
         if self.enckey is not None:
             pad_len = len(self.payload) % 16
             if pad_len > 0:
-                self.payload += bytes(16 - pad_len)
+                pad = bytes(16 - pad_len)
+                if isinstance(self.payload, bytes):
+                    self.payload += pad
+                else:
+                    self.payload.extend(pad)
 
         # This adds the header to the payload as well
         self.add_header(enckey, protected_tlv_size)