add a new SERIAL psm state, to ensure device doesn't sleep while

connected to the phone over USB.

In support of

https://github.com/meshtastic/Meshtastic-Android/issues/38

src/PowerFSM.h

@@ -14,6 +14,8 @@
 #define EVENT_NODEDB_UPDATED 8     // NodeDB has a big enough change that we think you should turn on the screen
 #define EVENT_CONTACT_FROM_PHONE 9 // the phone just talked to us over bluetooth
 #define EVENT_LOW_BATTERY 10 // Battery is critically low, go to sleep
+#define EVENT_SERIAL_CONNECTED 11
+#define EVENT_SERIAL_DISCONNECTED 12
 
 extern Fsm powerFSM;