add feather m0 exp

Author: ladyada

build_platform.py

@@ -54,6 +54,7 @@
     # Adafruit SAMD
     "gemma_m0" : "adafruit:samd:adafruit_gemma_m0",
     "trinket_m0" : "adafruit:samd:adafruit_trinket_m0",
+    "feather_m0_express" : "adafruit:samd:adafruit_feather_m0_express",
     "metro_m0" : "adafruit:samd:adafruit_metro_m0",
     "metro_m0_tinyusb" : "adafruit:samd:adafruit_metro_m0:usbstack=tinyusb",
     "metro_m4" : "adafruit:samd:adafruit_metro_m4:speed=120",