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"
