samd/mphalport: Add a timeout to mp_hal_stdout_tx_strn().

If USB CDC is connected and the board sends data, but the host does not
receive the data, the device locks up.  This is fixed in this commit by
having a timeout of 500ms, after which time the transmission is skipped.

ports/samd/mphalport.c

@@ -194,9 +194,14 @@ void mp_hal_stdout_tx_strn(const char *str, mp_uint_t len) {
             if (n > CFG_TUD_CDC_EP_BUFSIZE) {
                 n = CFG_TUD_CDC_EP_BUFSIZE;
             }
-            while (n > tud_cdc_write_available()) {
+            int timeout = 0;
+            // Wait with a max of USC_CDC_TIMEOUT ms
+            while (n > tud_cdc_write_available() && timeout++ < MICROPY_HW_USB_CDC_TX_TIMEOUT) {
                 MICROPY_EVENT_POLL_HOOK
             }
+            if (timeout >= MICROPY_HW_USB_CDC_TX_TIMEOUT) {
+                break;
+            }
             uint32_t n2 = tud_cdc_write(str + i, n);
             tud_cdc_write_flush();
             i += n2;

ports/samd/mphalport.h

@@ -35,6 +35,8 @@
 #include "hpl_time_measure.h"
 #include "sam.h"
 
+#define MICROPY_HW_USB_CDC_TX_TIMEOUT (500)
+
 extern int mp_interrupt_char;
 extern volatile uint32_t systick_ms;
 uint64_t mp_hal_ticks_us_64(void);