Add system halt handler.

cfg/chconf.h

@@ -787,6 +787,9 @@
  */
 #define CH_CFG_SYSTEM_HALT_HOOK(reason) {                                   \
   /* System halt code here.*/                                               \
+  while (*reason) { \
+    while ((USART1->SR & USART_SR_TXE) == 0) { __NOP(); } \
+    USART1->DR = *reason; reason ++; } \
 }
 
 /**