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, 0 deletions

diff --git a/supervisor/serial.h b/supervisor/serial.h
index 066886303..9c2d44737 100644
--- a/supervisor/serial.h
+++ b/supervisor/serial.h
@@ -47,4 +47,5 @@ char serial_read(void);
 bool serial_bytes_available(void);
 bool serial_connected(void);
 
+extern volatile bool _serial_connected;
 #endif  // MICROPY_INCLUDED_SUPERVISOR_SERIAL_H