Unify USB-related makefile var and C def as CIRCUITPY_USB.
Always define it as 0 or 1, same as all other settings.
USB_AVAILABLE was conditionally defined in supervisor.mk,
but never actually used to #ifdef USB-related code.
Loosely related to #4546
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)
