tools: fix idf_tools.py compatibility regression with Python 2

In Python 2, cadata must be explicitly converted to unicode:
https://bugs.python.org/issue37079

Regression from 081ebcf1e7776b4a67e3c350020068d66ce0e416

tools/idf_tools.py

@@ -143,7 +143,7 @@ CURRENT_PLATFORM = PLATFORM_FROM_NAME.get(PYTHON_PLATFORM, UNKNOWN_PLATFORM)
 EXPORT_SHELL = 'shell'
 EXPORT_KEY_VALUE = 'key-value'
 
-ISRG_X1_ROOT_CERT = """
+ISRG_X1_ROOT_CERT = u"""
 -----BEGIN CERTIFICATE-----
 MIIFazCCA1OgAwIBAgIRAIIQz7DSQONZRGPgu2OCiwAwDQYJKoZIhvcNAQELBQAw
 TzELMAkGA1UEBhMCVVMxKTAnBgNVBAoTIEludGVybmV0IFNlY3VyaXR5IFJlc2Vh