AP_SerialManager: added have_serial() method

this checks if we have the protocol without setting the options on the
uart

libraries/AP_SerialManager/AP_SerialManager.cpp

@@ -578,6 +578,12 @@ AP_HAL::UARTDriver *AP_SerialManager::find_serial(enum SerialProtocol protocol,
     return port;
 }
 
+// have_serial - return true if we have the given serial protocol configured
+bool AP_SerialManager::have_serial(enum SerialProtocol protocol, uint8_t instance) const
+{
+    return find_protocol_instance(protocol, instance) != nullptr;
+}
+
 // find_baudrate - searches available serial ports for the first instance that allows the given protocol
 //  instance should be zero if searching for the first instance, 1 for the second, etc
 //  returns baudrate on success, 0 if a serial port cannot be found

libraries/AP_SerialManager/AP_SerialManager.h

@@ -169,8 +169,12 @@ public:
     // find_serial - searches available serial ports that allows the given protocol
     //  instance should be zero if searching for the first instance, 1 for the second, etc
     //  returns uart on success, nullptr if a serial port cannot be found
+    // note that the SERIALn_OPTIONS are applied if the port is found
     AP_HAL::UARTDriver *find_serial(enum SerialProtocol protocol, uint8_t instance) const;
 
+    // have_serial - return true if we have the corresponding serial protocol configured
+    bool have_serial(enum SerialProtocol protocol, uint8_t instance) const;
+    
     // find_baudrate - searches available serial ports for the first instance that allows the given protocol
     //  instance should be zero if searching for the first instance, 1 for the second, etc
     //  returns the baudrate of that protocol on success, 0 if a serial port cannot be found