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/tos_message_queue.c

@@ -187,7 +187,6 @@ __API__ k_err_t tos_msg_q_pend(k_msg_q_t *msg_q, void **msg_ptr, k_tick_t timeou
     knl_sched();
 
     err = pend_state2errno(k_curr_task->pend_state);
-
     if (err == K_ERR_NONE) {
         *msg_ptr = k_curr_task->msg;
         k_curr_task->msg = K_NULL;