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/dev/arm/SConscript

@@ -63,7 +63,7 @@ SimObject('EnergyCtrl.py', sim_objects=['EnergyCtrl'], tags='arm isa')
 SimObject('NoMali.py', sim_objects=['NoMaliGpu', 'CustomNoMaliGpu'],
     enums=['NoMaliGpuType'], tags='arm isa')
 SimObject('VirtIOMMIO.py', sim_objects=['MmioVirtIO'], tags='arm isa')
-if env['USE_ARM_FASTMODEL']:
+if env['CONF']['USE_ARM_FASTMODEL']:
     SimObject('VExpressFastmodel.py', sim_objects=[], tags='arm isa')
 
 Source('a9scu.cc', tags='arm isa')