[BSP] use DoBuilding to replace env.Program/EndBuilding

bsp/frdm-k64f/SConstruct

@@ -30,8 +30,5 @@ Export('rtconfig')
 # prepare building environment
 objs = PrepareBuilding(env, RTT_ROOT, has_libcpu=False)
 
-# build program 
-env.Program(TARGET, objs)
-
-# end building 
-EndBuilding(TARGET)
+# do building 
+DoBuilding(TARGET, objs)