Integrate flaterm terminal

kernel/hal/x86_64/serial.c

@@ -1,6 +1,7 @@
 #include <stdint.h>
 #include <stdbool.h>
 #include "io.h"
+#include "putchar.h"
 
 #define SERIAL_PORT 0x3f8
 
@@ -42,7 +43,9 @@ bool serial_init(void) {
   return true;
 }
 
-// For printf library
-void putchar_(char c) {
-  serial_write(c);
-}
+#if PUTCHAR_ == PUTCHAR_SERIAL
+  // For printf library
+  void putchar_(char c) {
+    serial_write(c);
+  }
+#endif