Portenta: CM4: route Serial to UART1

Supersedes arduino#54

When we'll have full support for SerialRPC we can simply replace that line
(still need to understand how to notify the error in case CM7 is missing the "companion" code).

cores/arduino/Arduino.h

@@ -130,16 +130,13 @@ extern analogin_config_t adcCurrentConfig;
 #if defined(SERIAL_CDC)
 #include "USB/PluggableUSBSerial.h"
 #define Serial SerialUSB
+#else
+#define Serial _UART1_
+#endif
 #define Serial1 _UART1_
 #define Serial2 _UART2_
 #define Serial3 _UART3_
 #define Serial4 _UART4_
-#else
-#define Serial  _UART1_
-#define Serial1 _UART2_
-#define Serial2 _UART3_
-#define Serial3 _UART4_
-#endif
 
 #include "overloads.h"
 #endif