Fix object files listed multiple times in OBJS

[jepler]

While working with the Makefiles I noticed that some items are listed twice in $(OBJ). This doesn't usually pose a problem, but it seems unusual and would be nice to understand and fix. (It caused me a problem because I used $(OBJ) rather than $^ for placing the list of objects in a commandline)

During a build for feather_stm32f405_express, the following items appeared in $(OBJ) multiple times; the count is given.

      2 build-feather_stm32f405_express/shared-bindings/audiopwmio/__init__.o
      2 build-feather_stm32f405_express/shared-bindings/_bleio/Address.o
      2 build-feather_stm32f405_express/shared-bindings/_bleio/ScanEntry.o
      2 build-feather_stm32f405_express/shared-bindings/board/__init__.o
      2 build-feather_stm32f405_express/shared-bindings/canio/Match.o
      2 build-feather_stm32f405_express/shared-bindings/msgpack/__init__.o
      2 build-feather_stm32f405_express/shared-bindings/os/__init__.o
      2 build-feather_stm32f405_express/shared-bindings/rgbmatrix/__init__.o
      2 build-feather_stm32f405_express/shared-bindings/rgbmatrix/RGBMatrix.o
      3 build-feather_stm32f405_express/shared-bindings/_bleio/Attribute.o

[dhalbert]

$(sort will do this, for instance:

circuitpython/py/mkrules.mk

Lines 164 to 170 in 3236a0f

	# $(sort $(var)) removes duplicates
	#
	# The net effect of this, is it causes the objects to depend on the
	# object directories (but only for existence), and the object directories
	# will be created if they don't exist.
	OBJ_DIRS = $(sort $(dir $(OBJ)))
	$(OBJ): | $(OBJ_DIRS)