[ion] Handle USBEnumeration event like USBPlug.

Change-Id: I70c1dc92fdba8e56508a1dce65ad30a771326a30

ion/include/ion/usb.h

@@ -5,7 +5,10 @@ namespace Ion {
 namespace USB {
 
 bool isPlugged();
+bool isEnumerated(); // Speed-enumerated, to be accurate
+
 void removeSoftDisconnect();
+
 void DFU();
 
 }