change meme board MCU from 32u4 to 32u2 (#3212)

keyboards/meme/rules.mk

@@ -1,6 +1,6 @@
 # MCU name
 #MCU = at90usb1286
-MCU = atmega32u4
+MCU = atmega32u2
 
 # Processor frequency.
 #     This will define a symbol, F_CPU, in all source code files equal to the