Added sntp support and example

extras/sntp/sntp.h

@@ -0,0 +1,52 @@
+/************************************************************************//**
+ * RTC time keeping and synchronization using SNTP for esp-open-rtos.
+ * Uses SNTP lwIP contribution.
+ *
+ * 2016, Jesus Alonso (doragasu)
+ ****************************************************************************/
+
+#ifndef _SNTP_H_
+#define _SNTP_H_
+
+#include <stdint.h>
+#include <time.h>
+
+/// Update SNTP RTC timer
+void sntp_update_rtc(time_t t, uint32_t us);
+
+/// Function used to update the date/time, with microseconds resolution.
+#define SNTP_SET_SYSTEM_TIME_US(sec, us) sntp_update_rtc(sec, us)
+
+/// For the lwIP implementation of SNTP to allow using names for NTP servers.
+#define SNTP_SERVER_DNS             1
+
+/// Number of supported NTP servers
+#define SNTP_NUM_SERVERS_SUPPORTED 	4
+
+/// Initialize the module, and start requesting SNTP updates. This function
+/// must be called only once.
+void sntp_initialize(int time_zone, int day_light);
+
+/// Sets time zone. Allowed values are in the range [-11, 13].
+/// NOTE: Settings do not take effect until SNTP time is updated. It is
+void sntp_set_timezone(int time_zone);
+
+/// Sets daylight.
+/// NOTE: Settings do not take effect until SNTP time is updated.
+void sntp_set_daylight(int day_light);
+
+/// Returns the time read from RTC counter, in seconds from Epoch. If
+/// us is not null, it will be filled with the microseconds.
+time_t sntp_get_rtc_time(int32_t *us);
+
+/// Set SNTP servers. Up to SNTP_NUM_SERVERS_SUPPORTED can be set.
+/// Returns 0 if OK, less than 0 if error.
+/// NOTE: This function must NOT be called before sntp_initialize().
+int sntp_set_servers(char *server_url[], int num_servers);
+
+/// Sets the update delay in ms. If requested value is less than 15s,
+/// a 15s update interval will be set.
+void sntp_set_update_delay(uint32_t ms);
+
+#endif /* _SNTP_H_ */
+