shared/tinyusb: Allow ports to use 1200bps-touch without other CDC code.

This fixes the build for some esp32 and nrf boards (for example
`ARDUINO_NANO_33_BLE_SENSE` and `ARDUINO_NANO_ESP32`) due to commit
c98789a6d8.  Changes are:

- Allow the CDC TX/RX functions in `mp_usbd_cdc.c` to be enabled
  separately to those needed for `MICROPY_HW_USB_CDC_1200BPS_TOUCH`.

- Add `MICROPY_EXCLUDE_SHARED_TINYUSB_USBD_CDC` option as a temporary
  workaround for the nrf port to use.

- Declare `mp_usbd_line_state_cb()` in a header as a public function.

- Fix warning with type cast of `.callback_line_state_changed`.

Signed-off-by: Damien George <damien@micropython.org>

shared/tinyusb/mp_usbd_cdc.h

@@ -35,5 +35,8 @@ uintptr_t mp_usbd_cdc_poll_interfaces(uintptr_t poll_flags);
 void tud_cdc_rx_cb(uint8_t itf);
 mp_uint_t mp_usbd_cdc_tx_strn(const char *str, mp_uint_t len);
 
+#if MICROPY_HW_USB_EXTERNAL_TINYUSB
+void mp_usbd_line_state_cb(uint8_t itf, bool dtr, bool rts);
+#endif
 
 #endif // MICROPY_INCLUDED_SHARED_TINYUSB_MP_USBD_CDC_H