sys/arduino/sketches: force rebuild on sketch delete

Hack to force rebuild everytime so its rebuilt when a sketch is deleted.

There are different ways to properly fix it:

 * generate an equivalent of .d file included by make
 * change the sketches.cpp to directly include the .sketch files and GCC
   would do the job of generating the .d file with the '.sketch' files.

sys/arduino/arduino_sketches.inc.mk

@@ -29,3 +29,6 @@ $(SKETCH_MODULE_DIR):
 _ARDUINO_SKETCHES_MAKEFILE := $(lastword $(MAKEFILE_LIST))
 $(SKETCH_MODULE_DIR)/$(SKETCH_CPP): $(_ARDUINO_SKETCHES_MAKEFILE)
 $(SKETCH_MODULE_DIR)/Makefile: $(_ARDUINO_SKETCHES_MAKEFILE)
+
+# Hack to force rebuilding if one of the `SKETCHES` is deleted
+.PHONY: $(SKETCH_MODULE_DIR)/$(SKETCH_CPP)