include uf2 firmware for nrf52840 mdk usb dongle which has a uf2 bootloader now

Author: xiongyihui

tools/build_board_info.py

@@ -53,7 +53,7 @@
 
     # nRF52840 dev kits that may not have UF2 bootloaders,
     "makerdiary_nrf52840_mdk": HEX,
-    "makerdiary_nrf52840_mdk_usb_dongle": HEX,
+    "makerdiary_nrf52840_mdk_usb_dongle": HEX_UF2,
     "pca10056": BIN_UF2,
     "pca10059": BIN_UF2,
     "electronut_labs_blip": HEX