shared/tinyusb/mp_usbd_cdc: Skip writing to an uninitialized USB device.

During execution of `boot.py` the USB device is not yet initialized.  Any
attempt to write to the CDC (eg calling `print()`) would lock up the
device.  This commit skips writing when the USB device is not initialized.
Any output from `boot.py` is lost, but the device does not lock up.

Also removed unnecessary declaration of `tusb_init()`.

Signed-off-by: robert-hh <robert@hammelrath.com>

shared/tinyusb/mp_usbd.h

@@ -125,7 +125,6 @@ inline static bool mp_usb_device_builtin_enabled(const mp_obj_usb_device_t *usbd
 
 static inline void mp_usbd_init(void) {
     // Without runtime USB support, this can be a thin wrapper wrapper around tusb_init()
-    extern bool tusb_init(void);
     tusb_init();
 }
 

shared/tinyusb/mp_usbd_cdc.c

@@ -95,6 +95,9 @@ void tud_cdc_rx_cb(uint8_t itf) {
 }
 
 mp_uint_t mp_usbd_cdc_tx_strn(const char *str, mp_uint_t len) {
+    if (!tusb_inited()) {
+        return 0;
+    }
     size_t i = 0;
     while (i < len) {
         uint32_t n = len - i;