a bugfix for mail queue and printf clipping switch

1. add TOS_CFG_LIBC_PRINTF_EN
1: enable printf to implement tos_kprintf
when you call tos_kprintf(actually currently it's a MACRO rather than a function) , the libc will be invloved into the image, most of the time it means a larger image size.

0: disable printf, means tos_kprintf is defined as an empty MACRO
most of the time, means you will not get a log output from the uart(currently we donnot have a console yet) but a smaller image size.

2. fix a bug of tos_mail_q and tos_priority_mail_q
if we are post-ing a mail and there is[are] task[s] just pend-ing for the mail queue, the mail delivered to the pend-er will be a pointer, and probably it's a pointer to the local variable on the stack. Actually we should do a memcpy rather than a pointer assignment. for more details, see the patch.

kernel/core/include/tos_task.h

@@ -416,8 +416,9 @@ __KERNEL__ __STATIC_INLINE__ void task_state_set_sleeping(k_task_t *task)
 
 __DEBUG__ __STATIC_INLINE__ void task_default_walker(k_task_t *task)
 {
-    char *state_str;
+    char *state_str = "ABNORMAL";
 
+    state_str = state_str;
     tos_kprintln("tsk name: %s", task->name);
 
     if (task->state == K_TASK_STATE_PENDTIMEOUT_SUSPENDED) {