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_config_default.h

@@ -230,6 +230,10 @@
 #define  TOS_CFG_OBJECT_VERIFY_EN               0u
 #endif
 
+#ifndef TOS_CFG_LIBC_PRINTF_EN
+#define  TOS_CFG_LIBC_PRINTF_EN                     1u // we enable this by default
+#endif
+
 #if     (TOS_CFG_TIMER_AS_PROC == 0u) && !defined(TOS_CFG_TIMER_TASK_PRIO)
 #define  TOS_CFG_TIMER_TASK_PRIO            (k_prio_t)(K_TASK_PRIO_IDLE - (k_prio_t)1u)
 #endif