AP_HAL: added UART locking interface

used to gain exclusive access to a UART. This is needed for the BLHeli
MSP protocol to prevent protocol corruption due to console writes

libraries/AP_HAL/UARTDriver.h

@@ -37,6 +37,13 @@ public:
     virtual void set_blocking_writes(bool blocking) = 0;
     virtual bool tx_pending() = 0;
 
+    // lock a port for exclusive use. Use a key of 0 to unlock
+    virtual bool lock_port(uint32_t key) { return false; }
+
+    // write to a locked port. If port is locked and key is not correct then 0 is returned
+    // and write is discarded
+    virtual size_t write_locked(const uint8_t *buffer, size_t size, uint32_t key) { return 0; }
+    
     enum flow_control {
         FLOW_CONTROL_DISABLE=0, FLOW_CONTROL_ENABLE=1, FLOW_CONTROL_AUTO=2
     };