Fix SConstruct check for config.is_enabled()

SConstruct

@@ -192,13 +192,12 @@ for x in module_list:
     module_enabled = True
     tmppath = "./modules/" + x
     sys.path.append(tmppath)
-    try:
     import config
-        if (not config.is_enabled()):
+    enabled_attr = getattr(config, "is_enabled", None)
+    if (callable(enabled_attr) and not config.is_enabled()):
         module_enabled = False
-    except:
-        pass
     sys.path.remove(tmppath)
+    sys.modules.pop('config')
     opts.Add(BoolVariable('module_' + x + '_enabled', "Enable module '%s'" % (x, ), module_enabled))
 
 opts.Update(env_base)  # update environment