[msh] add available memory information for free cmd

Author: mysterywolf

components/finsh/msh.c

@@ -80,9 +80,10 @@ int cmd_free(int argc, char **argv)
     rt_size_t total = 0, used = 0, max_used = 0;
 
     rt_memory_info(&total, &used, &max_used);
-    rt_kprintf("total   : %d\n", total);
-    rt_kprintf("used    : %d\n", used);
-    rt_kprintf("maximum : %d\n", max_used);
+    rt_kprintf("total    : %d\n", total);
+    rt_kprintf("used     : %d\n", used);
+    rt_kprintf("maximum  : %d\n", max_used);
+    rt_kprintf("available: %d\n", total - used);
 #endif
     return 0;
 }