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/amdgpu/common/SConscript

@@ -31,10 +31,10 @@ import sys
 
 Import('*')
 
-if not env['BUILD_GPU']:
+if not env['CONF']['BUILD_GPU']:
     Return()
 
-if env['TARGET_GPU_ISA'] == 'gcn3' or env['TARGET_GPU_ISA'] == 'vega':
+if env['CONF']['TARGET_GPU_ISA'] in ('gcn3', 'vega'):
     SimObject('X86GPUTLB.py', sim_objects=['X86GPUTLB', 'TLBCoalescer'])
 
     Source('tlb.cc')

src/arch/amdgpu/gcn3/SConscript

@@ -33,10 +33,10 @@ import sys
 
 Import('*')
 
-if not env['BUILD_GPU']:
+if not env['CONF']['BUILD_GPU']:
     Return()
 
-if env['TARGET_GPU_ISA'] == 'gcn3':
+if env['CONF']['TARGET_GPU_ISA'] == 'gcn3':
     Source('decoder.cc')
     Source('insts/gpu_static_inst.cc')
     Source('insts/instructions.cc')

src/arch/amdgpu/vega/SConscript

@@ -33,7 +33,7 @@ import sys
 
 Import('*')
 
-if not env['BUILD_GPU']:
+if not env['CONF']['BUILD_GPU']:
     Return()
 
 SimObject('VegaGPUTLB.py', sim_objects=['VegaPagetableWalker',
@@ -48,7 +48,7 @@ Source('tlb_coalescer.cc')
 
 DebugFlag('GPUPTWalker', 'Debug flag for GPU page table walker')
 
-if env['TARGET_GPU_ISA'] == 'vega':
+if env['CONF']['TARGET_GPU_ISA'] == 'vega':
     Source('decoder.cc')
     Source('insts/gpu_static_inst.cc')
     Source('insts/instructions.cc')