Add support for custom defines

Author: qistoph

Arduino.mk

@@ -1083,6 +1083,10 @@ ifneq ($(CATERINA),)
     CPPFLAGS += -DUSB_VID=$(USB_VID) -DUSB_PID=$(USB_PID)
 endif
 
+ifdef CUSTOM_DEFINES
+	CPPFLAGS += $(CUSTOM_DEFINES)
+endif
+
 # avr-gcc version that we can do maths on
 CC_VERNUM = $(shell $(CC) -dumpversion | sed 's/\.//g')