esp32: Fixes issues discussed during code review of MR!341

The following issues mentioned during MR!341 review were fixed:
1) Core dump test application description
2) Usage of CONFIG_ESP32_ENABLE_COREDUMP_TO_FLASH and CONFIG_ESP32_ENABLE_COREDUMP_TO_UART
3) FLASH_GUARD_START macro usage is fixed in flash API
4) Core dump module logging facility
5) cache util functions doc updated
6) interactive delay before print core dump to uart
7) core dump partion support in build system

components/log/include/esp_log.h

@@ -75,6 +75,16 @@ void esp_log_set_vprintf(vprintf_like_t func);
  */
 uint32_t esp_log_timestamp(void);
 
+/**
+ * @brief Function which returns timestamp to be used in log output
+ *
+ * This function uses HW cycle counter and does not depend on OS,
+ * so it can be safely used after application crash.
+ *
+ * @return timestamp, in milliseconds
+ */
+uint32_t esp_log_early_timestamp(void);
+
 /**
  * @brief Write message into the log
  *