Fix(uart): no global serial creates linking error

cores/esp32/esp32-hal-uart.c

@@ -42,9 +42,6 @@
 static int s_uart_debug_nr = 0;         // UART number for debug output
 #define REF_TICK_BAUDRATE_LIMIT 250000  // this is maximum UART badrate using REF_TICK as clock
 
-/* C prototype for the notifier implemented in HardwareSerial.cpp */
-extern void hal_uart_notify_pins_detached(int uart_num);
-
 struct uart_struct_t {
 
 #if !CONFIG_DISABLE_HAL_LOCKS
@@ -296,7 +293,9 @@ static bool _uartDetachBus_RX(void *busptr) {
   }
   if (bus->_txPin < 0) {  // both rx and tx pins are detached, terminate the uart driver
     log_d("_uartDetachBus_RX: both RX and TX pins detached for UART%d, terminating driver", bus->num);
-    hal_uart_notify_pins_detached(bus->num);
+    if (hal_uart_notify_pins_detached) {
+      hal_uart_notify_pins_detached(bus->num);
+    }
     return true;
   }
   return _uartDetachPins(bus->num, bus->_rxPin, UART_PIN_NO_CHANGE, UART_PIN_NO_CHANGE, UART_PIN_NO_CHANGE);
@@ -315,7 +314,9 @@ static bool _uartDetachBus_TX(void *busptr) {
   }
   if (bus->_rxPin < 0) {  // both rx and tx pins are detached, terminate the uart driver
     log_d("_uartDetachBus_TX: both RX and TX pins detached for UART%d, terminating driver", bus->num);
-    hal_uart_notify_pins_detached(bus->num);
+    if (hal_uart_notify_pins_detached) {
+      hal_uart_notify_pins_detached(bus->num);
+    }
     return true;
   }
   return _uartDetachPins(bus->num, UART_PIN_NO_CHANGE, bus->_txPin, UART_PIN_NO_CHANGE, UART_PIN_NO_CHANGE);

cores/esp32/esp32-hal-uart.h

@@ -33,6 +33,9 @@ extern "C" {
 struct uart_struct_t;
 typedef struct uart_struct_t uart_t;
 
+// C prototype for the notifier that may be implemented in HardwareSerial.cpp
+extern void hal_uart_notify_pins_detached(int uart_num) __attribute__((weak));
+
 bool _testUartBegin(
   uint8_t uart_nr, uint32_t baudrate, uint32_t config, int8_t rxPin, int8_t txPin, uint32_t rx_buffer_size, uint32_t tx_buffer_size, bool inverted,
   uint8_t rxfifo_full_thrhd