Add create command to commands dict

`create` as alias to `sign` was defined in the arg parsing, but was not
functional because it was missing the subcommands call dict. This makes
`create` call the `sign` method.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/scripts/imgtool.py b/scripts/imgtool.py
index d4027e6..d6522d8 100755
--- a/scripts/imgtool.py
+++ b/scripts/imgtool.py
@@ -90,9 +90,11 @@
     img.save(args.outfile)
 
 subcmds = {
-        'keygen': do_keygen,
-        'getpub': do_getpub,
-        'sign': do_sign, }
+    'keygen': do_keygen,
+    'getpub': do_getpub,
+    'sign': do_sign,
+    'create': do_sign,
+}
 
 def alignment_value(text):
     value = int(text)