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>

src/cpu/simple/SConscript

@@ -28,7 +28,7 @@
 
 Import('*')
 
-if env['TARGET_ISA'] != 'null':
+if env['CONF']['TARGET_ISA'] != 'null':
     SimObject('BaseAtomicSimpleCPU.py', sim_objects=['BaseAtomicSimpleCPU'])
     Source('atomic.cc')