fix uf2 board id for samd51 boards

.. the previous ID was for samd21 microcontrollers.  the new id is the one for samd51 microcontrollers per https://github.com/microsoft/uf2/blob/master/utils/uf2families.json

Author: jepler

Diff for: build_platform.py

@@ -84,10 +84,10 @@
     "gemma_m0" : ["adafruit:samd:adafruit_gemma_m0", "0x68ed2b88"],
     "trinket_m0" : ["adafruit:samd:adafruit_trinket_m0", "0x68ed2b88"],
     "feather_m0_express" : ["adafruit:samd:adafruit_feather_m0_express", "0x68ed2b88"],
-    "feather_m4_express" : ["adafruit:samd:adafruit_feather_m4:speed=120", "0x68ed2b88"],
-    "feather_m4_express_tinyusb" : ["adafruit:samd:adafruit_feather_m4:speed=120,usbstack=tinyusb", "0x68ed2b88"],
-    "feather_m4_can" : ["adafruit:samd:adafruit_feather_m4_can:speed=120", "0x68ed2b88"],
-    "feather_m4_can_tinyusb" : ["adafruit:samd:adafruit_feather_m4_can:speed=120,usbstack=tinyusb", "0x68ed2b88"],
+    "feather_m4_express" : ["adafruit:samd:adafruit_feather_m4:speed=120", "0x55114460"],
+    "feather_m4_express_tinyusb" : ["adafruit:samd:adafruit_feather_m4:speed=120,usbstack=tinyusb", "0x55114460"],
+    "feather_m4_can" : ["adafruit:samd:adafruit_feather_m4_can:speed=120", "0x55114460"],
+    "feather_m4_can_tinyusb" : ["adafruit:samd:adafruit_feather_m4_can:speed=120,usbstack=tinyusb", "0x55114460"],
     "metro_m0" : ["adafruit:samd:adafruit_metro_m0", "0x68ed2b88"],
     "metro_m0_tinyusb" : ["adafruit:samd:adafruit_metro_m0:usbstack=tinyusb", "0x68ed2b88"],
     "metro_m4" : ["adafruit:samd:adafruit_metro_m4:speed=120", "0x55114460"],