add configTime, getLocalTime and support for time in Print

example:
```cpp
//done once on WiFi init
configTime(-7200, 3600, "pool.ntp.org");
//get local time
struct tm timeinfo;
if(!getLocalTime(&timeinfo)){
    Serial.println("Failed to obtain time");
    return;
}
//print time
Serial.println(&timeinfo);
//print time with different format
Serial.println(&timeinfo, "%A, %B %d %Y %H:%M:%S");
```
fixes: https://github.com/espressif/arduino-esp32/issues/29

cores/esp32/Arduino.h

@@ -162,6 +162,10 @@ uint16_t makeWord(byte h, byte l);
 unsigned long pulseIn(uint8_t pin, uint8_t state, unsigned long timeout = 1000000L);
 unsigned long pulseInLong(uint8_t pin, uint8_t state, unsigned long timeout = 1000000L);
 
+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);
+
 // WMath prototypes
 long random(long);
 #endif /* __cplusplus */

cores/esp32/Print.cpp

@@ -29,7 +29,7 @@
 
 #include "Print.h"
 extern "C" {
-//#include "esp_common.h"
+    #include "time.h"
 }
 
 // Public Methods //////////////////////////////////////////////////////////////
@@ -153,6 +153,18 @@ size_t Print::print(const Printable& x)
     return x.printTo(*this);
 }
 
+size_t Print::print(struct tm * timeinfo, const char * format)
+{
+    const char * f = format;
+    if(!f){
+        f = "%c";
+    }
+    char buf[64];
+    size_t written = strftime(buf, 64, f, timeinfo);
+    print(buf);
+    return written;
+}
+
 size_t Print::println(void)
 {
     return print("\r\n");
@@ -228,6 +240,13 @@ size_t Print::println(const Printable& x)
     return n;
 }
 
+size_t Print::println(struct tm * timeinfo, const char * format)
+{
+    size_t n = print(timeinfo, format);
+    n += println();
+    return n;
+}
+
 // Private Methods /////////////////////////////////////////////////////////////
 
 size_t Print::printNumber(unsigned long n, uint8_t base)

cores/esp32/Print.h

@@ -83,6 +83,7 @@ public:
     size_t print(unsigned long, int = DEC);
     size_t print(double, int = 2);
     size_t print(const Printable&);
+    size_t print(struct tm * timeinfo, const char * format = NULL);
 
     //size_t println(const __FlashStringHelper *);
     size_t println(const String &s);
@@ -95,6 +96,7 @@ public:
     size_t println(unsigned long, int = DEC);
     size_t println(double, int = 2);
     size_t println(const Printable&);
+    size_t println(struct tm * timeinfo, const char * format = NULL);
     size_t println(void);
 };
 

cores/esp32/esp32-hal-time.c

+// Copyright 2015-2016 Espressif Systems (Shanghai) PTE LTD
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "esp32-hal.h"
+#include "apps/sntp/sntp.h"
+
+static void setTimeZone(long offset, int daylight)
+{
+    char cst[16] = {0};
+    char cdt[16] = "CDT";
+    char tz[32] = {0};
+
+    if(offset % 3600){
+        sprintf(cst, "CST%ld:%02u:%02u", offset / 3600, abs((offset % 3600) / 60), abs(offset % 60));
+    } else {
+        sprintf(cst, "CST%ld", offset / 3600);
+    }
+    if(daylight != 3600){
+        long tz_dst = offset - daylight;
+        if(tz_dst % 3600){
+            sprintf(cdt, "CDT%ld:%02u:%02u", tz_dst / 3600, abs((tz_dst % 3600) / 60), abs(tz_dst % 60));
+        } else {
+            sprintf(cdt, "CDT%ld", tz_dst / 3600);
+        }
+    }
+    sprintf(tz, "%s%s", cst, cdt);
+    setenv("TZ", tz, 1);
+    tzset();
+}
+
+/*
+ * configTime
+ * Source: https://github.com/esp8266/Arduino/blob/master/cores/esp8266/time.c
+ * */
+void configTime(long gmtOffset_sec, int daylightOffset_sec, 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();
+    setTimeZone(gmtOffset_sec, daylightOffset_sec);
+}
+
+bool getLocalTime(struct tm * info, uint32_t ms)
+{
+    uint32_t count = ms / 10;
+    time_t now;
+
+    time(&now);
+    localtime_r(&now, info);
+
+    if(info->tm_year > (2016 - 1900)){
+        return true;
+    }
+
+    while(count--) {
+        delay(10);
+        time(&now);
+        localtime_r(&now, info);
+        if(info->tm_year > (2016 - 1900)){
+            return true;
+        }
+    }
+    return false;
+}
+