AP_HAL: make uart pointers private

this ensures they cannot be used by library or vehicle code, so we
will be able to remove them in future

libraries/AP_HAL/HAL.cpp

@@ -13,3 +13,16 @@ HAL::FunCallbacks::FunCallbacks(void (*setup_fun)(void), void (*loop_fun)(void))
 }
 
 }
+
+// access serial ports using SERIALn numbering
+AP_HAL::UARTDriver* AP_HAL::HAL::serial(uint8_t sernum) const
+{
+    UARTDriver **uart_array = const_cast<UARTDriver**>(&uartA);
+    // this mapping captures the historical use of uartB as SERIAL3
+    const uint8_t mapping[] = { 0, 2, 3, 1, 4, 5, 6, 7, 8 };
+    static_assert(sizeof(mapping) == num_serial, "num_serial must match mapping");
+    if (sernum >= num_serial) {
+        return nullptr;
+    }
+    return uart_array[mapping[sernum]];
+}

libraries/AP_HAL/HAL.h

@@ -101,6 +101,7 @@ class AP_HAL::HAL {
 
     virtual void run(int argc, char * const argv[], Callbacks* callbacks) const = 0;
 
+private:
     // the uartX ports must be contiguous in ram for the serial() method to work
     AP_HAL::UARTDriver* uartA;
     AP_HAL::UARTDriver* uartB;
@@ -111,6 +112,8 @@ class AP_HAL::HAL {
     AP_HAL::UARTDriver* uartG;
     AP_HAL::UARTDriver* uartH;
     AP_HAL::UARTDriver* uartI;
+
+public:
     AP_HAL::I2CDeviceManager* i2c_mgr;
     AP_HAL::SPIDeviceManager* spi;
     AP_HAL::AnalogIn*   analogin;
@@ -131,13 +134,7 @@ class AP_HAL::HAL {
 #endif
 
     // access to serial ports using SERIALn_ numbering
-    UARTDriver* serial(uint8_t sernum) const {
-        UARTDriver **uart_array = const_cast<UARTDriver**>(&uartA);
-        // this mapping captures the historical use of uartB as SERIAL3
-        const uint8_t mapping[] = { 0, 2, 3, 1, 4, 5, 6, 7, 8 };
-        if (sernum >= ARRAY_SIZE(mapping)) {
-            return nullptr;
-        }
-        return uart_array[mapping[sernum]];
-    }
+    UARTDriver* serial(uint8_t sernum) const;
+
+    static constexpr uint8_t num_serial = 9;
 };