[Keyboard] niu_mini uses dfu bootloader rather than the afrdude bootloader (#6092)

* niu_mini uses dfu bootloader rather than the afrdude bootloader
	modified: readme.md

* Change rules in rules.mk to reflect the bootloader change
	modified:   keyboards/niu_mini/rules.mk

keyboards/niu_mini/readme.md

@@ -11,6 +11,6 @@ Hardware Availability: [KBDFans](https://kbdfans.myshopify.com/products/niu-mini
 
 Make example for this keyboard (after setting up your build environment):
 
-    make niu_mini:default:avrdude
+    make niu_mini:default:dfu
 
 See [build environment setup](https://docs.qmk.fm/#/getting_started_build_tools) then the [make instructions](https://docs.qmk.fm/#/getting_started_make_guide) for more information.

keyboards/niu_mini/rules.mk

@@ -44,7 +44,7 @@ OPT_DEFS += -DINTERRUPT_CONTROL_ENDPOINT
 #   Atmel DFU loader 4096
 #   LUFA bootloader  4096
 #   USBaspLoader     2048
-OPT_DEFS += -DBOOTLOADER_SIZE=4096
+BOOTLOADER = atmel-dfu
 
 # Build Options
 #   change to "no" to disable the options, or define them in the Makefile in