Add new verify command

imgtool verify -k <some-key.(pub|sec)> <img-file>

Allow imgtool to validate that an image has a valid sha256sum and that
it was signed by the supplied key.

NOTE: this does not yet support verifying encrypted images

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/scripts/imgtool/keys/rsa.py b/scripts/imgtool/keys/rsa.py
index 94af064..0f9a905 100644
--- a/scripts/imgtool/keys/rsa.py
+++ b/scripts/imgtool/keys/rsa.py
@@ -62,6 +62,14 @@
     def sig_len(self):
         return self.key_size() / 8
 
+    def verify(self, signature, payload):
+        k = self.key
+        if isinstance(self.key, rsa.RSAPrivateKey):
+            k = self.key.public_key()
+        return k.verify(signature=signature, data=payload,
+                        padding=PSS(mgf=MGF1(SHA256()), salt_length=32),
+                        algorithm=SHA256())
+
 
 class RSA(RSAPublic):
     """