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/net/SConscript

@@ -44,7 +44,7 @@ SimObject('Ethernet.py', sim_objects=[
     'EtherLink', 'DistEtherLink', 'EtherBus', 'EtherSwitch', 'EtherTapBase',
     'EtherTapStub', 'EtherDump', 'EtherDevice', 'IGbE', 'EtherDevBase',
     'NSGigE', 'Sinic'] +
-    (['EtherTap'] if env['HAVE_TUNTAP'] else []))
+    (['EtherTap'] if env['CONF']['HAVE_TUNTAP'] else []))
 
 # Basic Ethernet infrastructure
 Source('etherbus.cc')