[Keyboard] Support flashing DZ60 with :flash command (#6624)

keyboards/dz60/rules.mk

@@ -36,10 +36,7 @@ F_USB = $(F_CPU)
 # Interrupt driven control endpoint task(+60)
 OPT_DEFS += -DINTERRUPT_CONTROL_ENDPOINT
 
-
-# Boot Section Size in *bytes*
-OPT_DEFS += -DBOOTLOADER_SIZE=4096
-
+BOOTLOADER=atmel-dfu
 
 # Build Options
 #   comment out to disable the options.