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)

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){

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

supervisor/shared/serial.c

@@ -47,6 +47,8 @@ busio_uart_obj_t debug_uart;
 byte buf_array[64];
 #endif
 
+volatile bool _serial_connected;
+
 void serial_early_init(void) {
 #if defined(DEBUG_UART_TX) && defined(DEBUG_UART_RX)
     debug_uart.base.type = &busio_uart_type;
@@ -69,7 +71,7 @@ bool serial_connected(void) {
 #if defined(DEBUG_UART_TX) && defined(DEBUG_UART_RX)
     return true;
 #else
-    return tud_cdc_connected();
+    return _serial_connected;
 #endif
 }
 

supervisor/shared/usb/usb.c

@@ -29,6 +29,7 @@
 #include "shared-module/usb_midi/__init__.h"
 #include "supervisor/background_callback.h"
 #include "supervisor/port.h"
+#include "supervisor/serial.h"
 #include "supervisor/usb.h"
 #include "lib/utils/interrupt_char.h"
 #include "lib/mp-readline/readline.h"
@@ -115,6 +116,7 @@ void tud_umount_cb(void) {
 // remote_wakeup_en : if host allows us to perform remote wakeup
 // USB Specs: Within 7ms, device must draw an average current less than 2.5 mA from bus
 void tud_suspend_cb(bool remote_wakeup_en) {
+    _serial_connected = false;
 }
 
 // Invoked when usb bus is resumed
@@ -126,6 +128,8 @@ void tud_resume_cb(void) {
 void tud_cdc_line_state_cb(uint8_t itf, bool dtr, bool rts) {
     (void) itf; // interface ID, not used
 
+    _serial_connected = dtr;
+
     // DTR = false is counted as disconnected
     if ( !dtr )
     {