supervisor: Improve serial connection detection

These changes remove the caveat from supervisor.runtime.serial_connected.

It appears that _tud_cdc_connected() only tracks explicit changes to the
"DTR" bit, which leads to disconnects not being registered.

Instead:
 * when line state is changed explicitly, track the dtr value in
   _serial_connected
 * when the USB bus is suspended, set _serial_connected to False

Testing performed (using sam e54 xplained):  Run a program to show
the state of `serial_connected` on the LED:
```
import digitalio
import supervisor
import board

led = digitalio.DigitalInOut(board.LED)
while True:
    led.switch_to_output(not supervisor.runtime.serial_connected)
```

Try all the following:
 * open, close serial terminal program
    - LED status tracks whether terminal is open
 * turn on/off data lines using the switchable charge-only cable
    - LED turns off when switch is in "charger" position
    - LED turns back on when switch is in Data position and terminal is
      opened (but doesn't turn back on just because switch position is
      changed)

1 files changed, 1 insertions, 7 deletions

diff --git a/shared-bindings/supervisor/Runtime.c b/shared-bindings/supervisor/Runtime.c
index 84d8370dd..bfca6e7b1 100755
--- a/shared-bindings/supervisor/Runtime.c
+++ b/shared-bindings/supervisor/Runtime.c
@@ -46,13 +46,7 @@
 //|
 
 //|     serial_connected: bool
-//|     """Returns the USB serial communication status (read-only).
-//|
-//|     .. note::
-//|
-//|         SAMD: Will return ``True`` if the USB serial connection
-//|         has been established at any point.  Will not reset if
-//|         USB is disconnected but power remains (e.g. battery connected)"""
+//|     """Returns the USB serial communication status (read-only)."""
 //|
 
 STATIC mp_obj_t supervisor_get_serial_connected(mp_obj_t self){