Added USB enable-disable mode (#25)

boot.py

@@ -0,0 +1,16 @@
+from board import *
+import digitalio
+import storage
+
+noStorageStatus = False
+noStoragePin = digitalio.DigitalInOut(GP15)
+noStoragePin.switch_to_input(pull=digitalio.Pull.UP)
+noStorageStatus = not noStoragePin.value
+
+if(noStorageStatus == True):
+    # don't show USB drive to host PC
+    storage.disable_usb_drive()
+    print("Disabling USB drive")
+else:
+    # normal boot
+    print("USB drive enabled")