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/proto/SConscript

@@ -38,14 +38,8 @@
 Import('*')
 
 # Only build if we have protobuf support
-if env['HAVE_PROTOBUF']:
-    ProtoBuf('inst_dep_record.proto')
-    ProtoBuf('packet.proto')
-    ProtoBuf('inst.proto')
-    Source('protobuf.cc')
-    Source('protoio.cc')
-
-    # protoc relies on the fact that undefined preprocessor symbols are
-    # explanded to 0 but since we use -Wundef they end up generating
-    # warnings.
-    env.Append(CCFLAGS='-DPROTOBUF_INLINE_NOT_IN_HEADERS=0')
+ProtoBuf('inst_dep_record.proto', tags='protobuf')
+ProtoBuf('packet.proto', tags='protobuf')
+ProtoBuf('inst.proto', tags='protobuf')
+Source('protobuf.cc', tags='protobuf')
+Source('protoio.cc', tags='protobuf')