Improve RUI3 AT command compatibility

• Add AT+FIRMWAREVER command
• Add option to set application version with

snprintf(g_custom_fw_ver,63,"RAK11200 ENV V%d.%d.%d",SW_VERSION_1,SW_VERSION_2,SW_VERSION_3);

AT+VER=? command will show WisBlock API V2 version
AT+FIRMWAREVER=? command will show application version. Defaults to "Custom firmware"

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• Add AT+SYNCWORD command
  AT+SYNCWORD=? will show current sync word
  AT+SYNCWORD=244B sets sync word to 244B