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/arch/arm/kvm/SConscript

@@ -40,7 +40,7 @@ Import('*')
 import platform
 host_isa = platform.machine()
 
-if not (env['USE_KVM'] and env['KVM_ISA'] == 'arm'):
+if not (env['CONF']['USE_KVM'] and env['CONF']['KVM_ISA'] == 'arm'):
     Return()
 
 SimObject('KvmGic.py',