waf: use Configure.autoconfig = 'clobber'

That makes reconfiguration be automatically triggered when wscripts used in
configuration are changed.

wscript

@@ -10,7 +10,9 @@ sys.path.insert(0, 'Tools/ardupilotwaf/')
 import ardupilotwaf
 import boards
 
-from waflib import Build, ConfigSet, Context, Utils
+from waflib import Build, ConfigSet, Configure, Context, Utils
+
+Configure.autoconfig = 'clobber'
 
 # TODO: implement a command 'waf help' that shows the basic tasks a
 # developer might want to do: e.g. how to configure a board, compile a