Keyboard: Support PCBDOWN variants for Atreus keyboard (#3735)

keyboards/atreus/readme.md

@@ -16,4 +16,6 @@ Make example for this keyboard (after setting up your build environment):
 Unlike the TMK firmware, this command should be run from the root of
 the repository, not the directory containing this readme.
 
+If your keyboard layout is a mirror image of what you expected (i.e. you do not get QWERTY on the left but YTREWQ on the right), then you have an A-Star powered Atreus (older than March 2016) with PCB labels facing *down* instead of up. Specify that by adding `PCBDOWN=yes` to your `make` commands.
+
 See [build environment setup](https://docs.qmk.fm/build_environment_setup.html) then the [make instructions](https://docs.qmk.fm/make_instructions.html) for more information.

keyboards/atreus/rules.mk

@@ -5,6 +5,9 @@ ifdef TEENSY2
     ATREUS_UPLOAD_COMMAND = teensy_loader_cli -w -mmcu=$(MCU) $(TARGET).hex
 else
     OPT_DEFS += -DATREUS_ASTAR
+ifdef PCBDOWN
+    OPT_DEFS += -DPCBDOWN
+endif
     ATREUS_UPLOAD_COMMAND = while [ ! -r $(USB) ]; do sleep 1; done; \
                             avrdude -p $(MCU) -c avr109 -U flash:w:$(TARGET).hex -P $(USB)
 endif