feature: kalloc_frame and demo in the main

src/kernel.c

@@ -1,6 +1,8 @@
 #include "gdt.h"
+#include "kprintf.h"
 #include "memory.h"
 #include "shell.h"
+#include "string.h"
 #include "terminal.h"
 
 #include <stdbool.h>
@@ -23,5 +25,8 @@ void kernel_main(void)
 	terminal_initialize();
 	init_gdt();
 	init_memory();
+	char *str = (char *)kalloc_frame(1);
+	memcpy(str, "Hello world!\n", 15);
+	kprintf(KERN_INFO, "%s", str);
 	shell_init();
 }