misc: Text vs Byte string in python3

Python 3 uses the concepts of two different types:
text and binary strings.
Those cannot be implicilty combined (as it was happening in python2) and
in order to be used together one of them must be converted to the other
type:

* Text can be encoded into Bytes via the encode() method
* Bytes can be decoded to Text using the decode() method

By default encode/decode will assume UTF-8 format

JIRA: https://gem5.atlassian.net/browse/GEM5-345

Change-Id: I1bdf7db17b49cc109239fd5f44791769370853f8
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26250
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>

SConstruct

@@ -1013,7 +1013,7 @@ export_vars += ['USE_FENV', 'TARGET_ISA', 'TARGET_GPU_ISA', 'CP_ANNOTATE',
 # operands are the name of the variable and a Value node containing the
 # value of the variable.
 def build_config_file(target, source, env):
-    (variable, value) = [s.get_contents() for s in source]
+    (variable, value) = [s.get_contents().decode('utf-8') for s in source]
     with open(str(target[0]), 'w') as f:
         print('#define', variable, value, file=f)
     return None