add_upgrade_boot_entry: make sure to not expand unicode

repos/system_upgrade/common/actors/addupgradebootentry/libraries/addupgradebootentry.py

@@ -59,12 +59,12 @@ def fmt_single_arg(arg_pair):
     def flatten_arguments(arg_pair):
         """ Expand multi-valued values into an iterable (key, value1), (key, value2) """
         key, value = arg_pair
-        if isinstance(value, str) or value is None:
-            yield (key, value)  # Just a single (key, value) pair
-        else:
+        if isinstance(value, (tuple, list)):
             # value is multi-valued (a tuple of values)
             for value_elem in value:  # yield from is not available in python2.7
                 yield (key, value_elem)
+        else:
+            yield (key, value)  # Just a single (key, value) pair
 
     arg_sequence = itertools.chain(*(flatten_arguments(arg_pair) for arg_pair in args_dict.items()))