show_config_variable
In 5ed507cf43 as part of #98, [`show_config_variable` was duplicated in Arduino.mk file](https://github.com/sudar/Arduino-Makefile/pull/98#commitcomment-3561542) But it was later found that `ARDMK_DIR` is still not getting printed (issue #111). This commit therefore removes the duplicated macro definition.
5ed507cf43
ARDMK_DIR