add configTzTime() to setup sntp using TZ environment variable (#608)

cores/esp32/Arduino.h

@@ -168,6 +168,8 @@ unsigned long pulseInLong(uint8_t pin, uint8_t state, unsigned long timeout = 10
 extern "C" bool getLocalTime(struct tm * info, uint32_t ms = 5000);
 extern "C" void configTime(long gmtOffset_sec, int daylightOffset_sec,
         const char* server1, const char* server2 = nullptr, const char* server3 = nullptr);
+extern "C" void configTzTime(const char* tz,
+        const char* server1, const char* server2 = nullptr, const char* server3 = nullptr);
 
 // WMath prototypes
 long random(long);

cores/esp32/esp32-hal-time.c

@@ -45,7 +45,6 @@ static void setTimeZone(long offset, int daylight)
  * */
 void configTime(long gmtOffset_sec, int daylightOffset_sec, const char* server1, const char* server2, const char* server3)
 {
-
     if(sntp_enabled()){
         sntp_stop();
     }
@@ -57,6 +56,24 @@ void configTime(long gmtOffset_sec, int daylightOffset_sec, const char* server1,
     setTimeZone(gmtOffset_sec, daylightOffset_sec);
 }
 
+/*
+ * configTzTime
+ * sntp setup using TZ environment variable
+ * */
+void configTzTime(const char* tz, const char* server1, const char* server2, const char* server3)
+{
+    if(sntp_enabled()){
+        sntp_stop();
+    }
+    sntp_setoperatingmode(SNTP_OPMODE_POLL);
+    sntp_setservername(0, (char*)server1);
+    sntp_setservername(1, (char*)server2);
+    sntp_setservername(2, (char*)server3);
+    sntp_init();
+    setenv("TZ", tz, 1);
+    tzset();
+}
+
 bool getLocalTime(struct tm * info, uint32_t ms)
 {
     uint32_t count = ms / 10;