[msh] add available memory information for free cmd

components/finsh/msh.c

@@ -83,6 +83,7 @@ int cmd_free(int argc, char **argv)
     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;
 }