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

docs/software/power.md

@@ -36,6 +36,10 @@ From lower to higher power consumption.
   onEntry: setBluetoothOn(true), screen.setOn(true)
   onExit: screen.setOn(false)
 
+- serial API usage (SERIAL) - Screen is on, device doesn't sleep, bluetooth off
+  onEntry: setBluetooth off, screen on
+  onExit:
+
 ## Behavior
 
 ### events that increase CPU activity
@@ -51,9 +55,11 @@ From lower to higher power consumption.
 - While in DARK/ON: If we receive EVENT_BLUETOOTH_PAIR we transition to ON and start our screen_on_secs timeout
 - While in NB/DARK/ON: If we receive EVENT_NODEDB_UPDATED we transition to ON (so the new screen can be shown)
 - While in DARK: While the phone talks to us over BLE (EVENT_CONTACT_FROM_PHONE) reset any sleep timers and stay in DARK (needed for bluetooth sw update and nice user experience if the user is reading/replying to texts)
+- while in LS/NB/DARK: if SERIAL_CONNECTED, go to serial
 
 ### events that decrease cpu activity
 
+- While in SERIAL: if SERIAL_DISCONNECTED, go to NB
 - While in ON: If PRESS event occurs, reset screen_on_secs timer and tell the screen to handle the pess
 - While in ON: If it has been more than screen_on_secs since a press, lower to DARK
 - While in DARK: If time since last contact by our phone exceeds phone_timeout_secs (15 minutes), we transition down into NB mode