HAL and serial cleanup

Co-Authored-By: Jason Smith <20053467+sjasonsmith@users.noreply.github.com>

Marlin/src/HAL/LINUX/main.cpp

@@ -107,7 +107,7 @@ int main() {
   std::thread write_serial (write_serial_thread);
   std::thread read_serial (read_serial_thread);
 
-  #if NUM_SERIAL > 0
+  #ifdef MYSERIAL0
     MYSERIAL0.begin(BAUDRATE);
     SERIAL_ECHOLNPGM("x86_64 Initialized");
     SERIAL_FLUSHTX();