imgtool: Fix getpub fails for ed25519 key

Fix getpub command fails to emit public pair of ed25519 key with
encoding.

Signed-off-by: Rustam Ismayilov <rustam.ismayilov@arm.com>
Change-Id: I3d255f576b2d8a8d0cbc8e31ff378717acb060c7

scripts/imgtool/keys/ed25519.py

@@ -4,7 +4,6 @@ ED25519 key management
 
 # SPDX-License-Identifier: Apache-2.0
 
-from cryptography.hazmat.backends import default_backend
 from cryptography.hazmat.primitives import serialization
 from cryptography.hazmat.primitives.asymmetric import ed25519
 
@@ -34,6 +33,11 @@ class Ed25519Public(KeyClass):
                 encoding=serialization.Encoding.DER,
                 format=serialization.PublicFormat.SubjectPublicKeyInfo)
 
+    def get_public_pem(self):
+        return self._get_public().public_bytes(
+            encoding=serialization.Encoding.PEM,
+            format=serialization.PublicFormat.SubjectPublicKeyInfo)
+
     def get_private_bytes(self, minimal, format):
         self._unsupported('get_private_bytes')