esp32/machine_uart: Change sendbreak time to be at least 15 bit times.

It used to be 10 bit times, which is too short.  The break state must be
longer than a regular character time, at least 13 bit times.  This is now
implemented by reducing the baudrate while sending the "0".  The break time
will now vary with data length and parity setting, but will at least be 15
bit times.

Tested with a GENERIC_SPIRAM, GENERIC_C3 and UM_TINYS2 board.

ports/esp32/machine_uart.c

@@ -399,28 +399,19 @@ STATIC mp_obj_t machine_uart_sendbreak(mp_obj_t self_in) {
     machine_uart_obj_t *self = MP_OBJ_TO_PTR(self_in);
 
     // Save settings
-    uart_word_length_t word_length;
-    uart_parity_t parity;
-    uart_get_word_length(self->uart_num, &word_length);
-    uart_get_parity(self->uart_num, &parity);
+    uint32_t baudrate;
+    uart_get_baudrate(self->uart_num, &baudrate);
 
-    // Synthesise the break condition by either a longer word or using even parity
+    // Synthesise the break condition by reducing the baud rate,
+    // and cater for the worst case of 5 data bits, no parity.
     uart_wait_tx_done(self->uart_num, pdMS_TO_TICKS(1000));
-    if (word_length != UART_DATA_8_BITS) {
-        uart_set_word_length(self->uart_num, UART_DATA_8_BITS);
-    } else if (parity == UART_PARITY_DISABLE) {
-        uart_set_parity(self->uart_num, UART_PARITY_EVEN);
-    } else {
-        // Cannot synthesise break
-        mp_raise_OSError(MP_EPERM);
-    }
+    uart_set_baudrate(self->uart_num, baudrate * 6 / 15);
     char buf[1] = {0};
     uart_write_bytes(self->uart_num, buf, 1);
     uart_wait_tx_done(self->uart_num, pdMS_TO_TICKS(1000));
 
-    // Restore original settings
-    uart_set_word_length(self->uart_num, word_length);
-    uart_set_parity(self->uart_num, parity);
+    // Restore original setting
+    uart_set_baudrate(self->uart_num, baudrate);
 
     return mp_const_none;
 }