mbed_localtime: Add support of wday.

This field is necessary, it is used by several vendor RTC: Atmel, ST, NUVOTON,
NXP.

platform/mbed_mktime.c

@@ -123,6 +123,11 @@ bool _rtc_localtime(time_t timestamp, struct tm* time_info) {
     time_info->tm_hour = timestamp % 24;
     timestamp = timestamp / 24;  // timestamp in days;
 
+    // compute the weekday
+    // The 1st of January 1970 was a Thursday which is equal to 4 in the weekday
+    // representation ranging from [0:6]
+    time_info->tm_wday = (timestamp + 4) % 7;
+
     // years start at 70
     time_info->tm_year = 70;
     while (true) { 

platform/mbed_mktime.h

@@ -80,6 +80,7 @@ time_t _rtc_mktime(const struct tm* calendar_time);
  *   - tm_mday
  *   - tm_mon
  *   - tm_year
+ *   - tm_wday
  * The object remains untouched if the time in input is invalid.
  * @return true if the conversion was successful, false otherwise.
  *