docs(secure_boot_v2): Mention idf.py and openssl commands to generate and verify signatures

- Adds support for verify_signature command in idf.py

Closes https://github.com/espressif/esptool/issues/942

tools/test_idf_py/test_idf_py.py

@@ -521,6 +521,19 @@ class TestSecureCommands(TestWrapperCommands):
         output = self.call_command(sign_command)
         self.assertIn('Signed', output)
 
+    def secure_verify_signature(self):
+        self.secure_sign_data()
+        sign_command = [sys.executable,
+                        idf_py_path,
+                        'secure-verify-signature',
+                        '--version',
+                        '2',
+                        '--keyfile',
+                        f'../{self.signing_key}',
+                        'bootloader-signed.bin']
+        output = self.call_command(sign_command)
+        self.assertIn('verification successful', output)
+
 
 class TestMergeBinCommands(TestWrapperCommands):
     """