scons: Add a pair of functions for working with Value nodes.

These nodes are really for working with text, and they do strange and
broken things otherwise. This change adds a pair of convenience
functions which uses pickle to serialize an object for the Value to
hold, and to unpack the Value later.

Change-Id: Id48822ce3de283b003d4cc7ef6b563a70e321ca6
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48371
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>

site_scons/gem5_scons/__init__.py

@@ -39,6 +39,7 @@
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 import os
+import pickle
 import sys
 import tempfile
 import textwrap
@@ -46,6 +47,7 @@ import textwrap
 from gem5_scons.util import get_termcap
 from gem5_scons.configure import Configure
 from gem5_scons.defaults import EnvDefaults
+import SCons.Node.Python
 import SCons.Script
 
 termcap = get_termcap()
@@ -260,5 +262,11 @@ else:
         env['SHCCCOMSTR']      = Transform("SHCC")
         env['SHCXXCOMSTR']     = Transform("SHCXX")
 
+def ToValue(obj):
+    return SCons.Node.Python.Value(pickle.dumps(obj))
+
+def FromValue(node):
+    return pickle.loads(node.read())
+
 __all__ = ['Configure', 'EnvDefaults', 'Transform', 'warning', 'error',
-           'MakeAction', 'MakeActionTool']
+           'MakeAction', 'MakeActionTool', 'ToValue', 'FromValue']