Build: fix case of "com3" and add Arduino path override

mk/configure.mk

@@ -10,10 +10,13 @@ configure:
 	@echo  >> $(SKETCHBOOK)/config.mk
 	@echo \# The communication port used to communicate with the APM. >> $(SKETCHBOOK)/config.mk
 ifneq ($(findstring CYGWIN, $(SYSTYPE)),)
-	@echo PORT = com3 >> $(SKETCHBOOK)/config.mk
+	@echo PORT = COM3 >> $(SKETCHBOOK)/config.mk
 else
 	@echo PORT = /dev/ttyACM0 >> $(SKETCHBOOK)/config.mk
 endif
+	@echo  >> $(SKETCHBOOK)/config.mk
+	@echo  \# uncomment and fill in the path to Arduino if installed in an exotic location >> $(SKETCHBOOK)/config.mk
+	@echo  \# ARDUINO = /path/to/Arduino  >> $(SKETCHBOOK)/config.mk
 	@echo  >> $(SKETCHBOOK)/config.mk
 	@echo \# PX4 app build: uncomment and fill in the path to PX4 Firmware repository: >> $(SKETCHBOOK)/config.mk
 	@echo \# PX4_ROOT = /path/to/Firmware >> $(SKETCHBOOK)/config.mk