Clean up special serial port cases

Assume SERIAL_PORT 0 if not defined. Collapse un-numbered register
special case. Clarify comments.

Marlin/MarlinSerial.h

 #include "Marlin.h"
 
 #if !defined(SERIAL_PORT) 
-#error SERIAL_PORT not set 
+#define SERIAL_PORT 0
 #endif
 
 // The presence of the UBRRH register is used to detect a UART.
 // These are macros to build serial port register names for the selected SERIAL_PORT (C preprocessor
 // requires two levels of indirection to expand macro values properly)
 #define SERIAL_REGNAME(registerbase,number,suffix) SERIAL_REGNAME_INTERNAL(registerbase,number,suffix)
-#if SERIAL_PORT == 0 && !defined(UBRR0H)
+#if SERIAL_PORT == 0 && (!defined(UBRR0H) || !defined(UDR0)) // use un-numbered registers if necessary
 #define SERIAL_REGNAME_INTERNAL(registerbase,number,suffix) registerbase##suffix
-#else
+#else
 #define SERIAL_REGNAME_INTERNAL(registerbase,number,suffix) registerbase##number##suffix
-#endif
+#endif
 
 // Registers used by MarlinSerial class (these are expanded 
 // depending on selected serial port
-#define M_UCSRxA SERIAL_REGNAME(UCSR,SERIAL_PORT,A) // defines M_UCSRxA to be UCSRxA where x is the serial port number
+#define M_UCSRxA SERIAL_REGNAME(UCSR,SERIAL_PORT,A) // defines M_UCSRxA to be UCSRnA where n is the serial port number
 #define M_UCSRxB SERIAL_REGNAME(UCSR,SERIAL_PORT,B) 
 #define M_RXENx SERIAL_REGNAME(RXEN,SERIAL_PORT,)    
 #define M_TXENx SERIAL_REGNAME(TXEN,SERIAL_PORT,)    
 #define M_RXCIEx SERIAL_REGNAME(RXCIE,SERIAL_PORT,)    
 #define M_UDREx SERIAL_REGNAME(UDRE,SERIAL_PORT,)    
-#if SERIAL_PORT == 0 && !defined(UDR0)
-  #if defined(UDR)
-    #define M_UDRx UDR  //  atmega8, atmega32
-  #else
-    #error UDR not defined
-  #endif
-#else
-  #define M_UDRx SERIAL_REGNAME(UDR,SERIAL_PORT,) 
-#endif  
+#define M_UDRx SERIAL_REGNAME(UDR,SERIAL_PORT,)  
 #define M_UBRRxH SERIAL_REGNAME(UBRR,SERIAL_PORT,H)
 #define M_UBRRxL SERIAL_REGNAME(UBRR,SERIAL_PORT,L)
 #define M_RXCx SERIAL_REGNAME(RXC,SERIAL_PORT,)
 extern MarlinSerial MSerial;
 #endif // ! teensylu
 
-#endif
+#endif