added sending unconfirmed message for RHF76

1. added AT+ command "AT+MSGHEX" and change previous RHF76_ATCMD_FMT_SEND_MSGHEX to RHF76_ATCMD_FMT_SEND_CMSGHEX

2. implement send_unconfirmed with a non-breaking approach: added int (*send_unconfirmed)(const void *buf, size_t len); event handle in lora_module_st; added int tos_lora_module_send_unconfirmed(const void *buf, size_t len); in lora_module_wrapper.c; and implement static int rhf76_send_unconfirmed(const void *buf, size_t len) in RHF76.c

3. added { "+MSGHEX: PORT:", rhf76_incoming_data_process } in event array rhf76_at_event so that the device is able to process the downlink data when it is configurated to send unconfirmed message.

4. added AT+ command "AT+REPT", and implement the static int rhf76_set_repeat(uint8_t num) to enable user to set repeat times (1-15) while using "unconfirmed message"

5. updated the instructions of implemeting up-link parser and downlink parser on the server in the file "..\board\NUCLEO_STM32L073RZ\BSP\Src\lora_demo.c"

net/lora_module_wrapper/lora_module_wrapper.c

@@ -44,6 +44,14 @@ int tos_lora_module_send(const void *buf, size_t len)
     return -1;
 }
 
+int tos_lora_module_send_unconfirmed(const void *buf, size_t len)
+{
+    if (g_lora_module && g_lora_module->send_unconfirmed) {
+        return g_lora_module->send_unconfirmed(buf, len);
+    }
+    return -1;
+}
+
 int tos_lora_module_recvcb_register(lora_recv_callback_t recv_callback)
 {
     if (g_lora_module) {

net/lora_module_wrapper/lora_module_wrapper.h

@@ -32,6 +32,8 @@ typedef struct lora_module_st {
 
     int (*send)(const void *buf, size_t len);
 
+    int (*send_unconfirmed)(const void *buf, size_t len);
+    
     int (*close)(void);
 
     lora_recv_callback_t recv_callback;
@@ -87,6 +89,18 @@ int tos_lora_module_join_abp(const char *deveui, const char *devaddr, const char
  */
 int tos_lora_module_send(const void *buf, size_t len);
 
+/**
+ * @brief Send unconfirmed data (message) by lora module.
+ *
+ * @attention None
+ *
+ * @param[in]   buf     data to send
+ * @param[in]   len     length of the data
+ *
+ * @return  errcode
+ */
+int tos_lora_module_send_unconfirmed(const void *buf, size_t len);
+
 /**
  * @brief Register a Receive callback method by lora module.
  *