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/systemc/SConsopts

@@ -44,5 +44,7 @@ def use_systemc_check(env, warn=False):
 
 main.AddMethod(use_systemc_check, 'UseSystemcCheck')
 
+main['CONF']['USE_SYSTEMC'] = main.UseSystemcCheck()
+
 sticky_vars.Add(BoolVariable('USE_SYSTEMC', 'Enable SystemC API support',
                              main.UseSystemcCheck()))