scons: Put all config variables in an env['CONF'] sub-dict.

This makes what are configuration and what are internal SCons variables
explicit and separate, and makes it unnecessary to call out what
variables to export to C++.

These variables will also be plumbed into and out of kconfiglib in later
changes.

Change-Id: Iaf5e098d7404af06285c421dbdf8ef4171b3f001
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/56892
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

site_scons/gem5_scons/builders/config_file.py

@@ -67,7 +67,7 @@ def ConfigFile(env):
         variable = str(target[0])
         # True target is config header file
         target = env.Dir('config').File(variable.lower() + '.hh')
-        val = env[variable]
+        val = env['CONF'][variable]
         if isinstance(val, bool):
             # Force value to 0/1
             val = int(val)