scons: Make all sticky variables automatically exported.

All sticky vars are exported, but not all exported vars are sticky. The
vars which are exported but not sticky are (at least in general) found
with Configure() style measurement.

Change-Id: Idebf17e44c2eeca745cdfdd9f42eddcfdb0cf9ed
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/56891
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>

src/SConscript

@@ -60,7 +60,11 @@ Import('*')
 # Children need to see the environment
 Export('env')
 
-build_env = [(opt, env[opt]) for opt in export_vars]
+all_export_vars = set()
+all_export_vars.update(export_vars)
+all_export_vars.update(sticky_vars.keys())
+
+build_env = [(opt, env[opt]) for opt in all_export_vars]
 
 from code_formatter import code_formatter
 
@@ -533,7 +537,7 @@ for extra_dir in extras_dir_list:
             build_dir = os.path.join(env['BUILDDIR'], root[prefix_len:])
             SConscript(os.path.join(root, 'SConscript'), variant_dir=build_dir)
 
-for opt in export_vars:
+for opt in all_export_vars:
     env.ConfigFile(opt)
 
 def makeTheISA(source, target, env):