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/cpu/kvm/SConsopts

@@ -76,7 +76,7 @@ if main['KVM_ISA']:
                 'Enable hardware virtualized (KVM) CPU models', True))
 else:
     main['USE_KVM'] = False
+    export_vars.append('USE_KVM')
     warning("Can not enable KVM, host seems to lack KVM support")
 
-export_vars.append('USE_KVM')
 export_vars.append('KVM_ISA')