Set Bootloader Size to 4K

Use Atmel's default bootloader size, rather than Teensy 2.0's [1].

[1] https://github.com/qmk/qmk_firmware/pull/1207#discussion_r109712786

keyboards/miuni32/rules.mk

@@ -45,7 +45,7 @@ OPT_DEFS += -DINTERRUPT_CONTROL_ENDPOINT
 #   Atmel DFU loader 4096
 #   LUFA bootloader  4096
 #   USBaspLoader     2048
-OPT_DEFS += -DBOOTLOADER_SIZE=512
+OPT_DEFS += -DBOOTLOADER_SIZE=4096
 
 
 # Build Options