Replace CLI serial port configuration code with a new serial command.

This is done to make it clearer in configuration dumps which ports are
enabled and configured.

There is a nice code size reduction due to less strings being needed.

Serial documentation updated.

Closes #966.

src/main/io/serial.c

@@ -89,7 +89,7 @@ baudRate_e lookupBaudRateIndex(uint32_t baudRate)
             return index;
         }
     }
-    return 0;
+    return BAUD_AUTO;
 }
 
 static serialPortUsage_t *findSerialPortUsageByIdentifier(serialPortIdentifier_e identifier)