feat(esp8266): add system version

Bootloader can get the version of application for specific section,
and then check if some features are supported.

Developers can use the macro "ESP_IDF_VERSION" to limite some function like following:

include "esp_idf_version.h"

if ESP_IDF_VERSION >= ESP_IDF_VERSION_VAL(3, 4, 0) && ESP_IDF_VERSION < ESP_IDF_VERSION_VAL(4, 0, 0) // 3.4 <= ver < 4.0
    do_xxx_process();
endif

components/esp8266/include/esp_system.h

@@ -21,6 +21,8 @@
 
 #include "sdkconfig.h"
 
+#include "esp_idf_version.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -181,13 +183,6 @@ uint32_t esp_get_minimum_free_heap_size( void );
  */
 uint32_t esp_random(void);
 
-/**
- * Get IDF version
- *
- * @return constant string from IDF_VER
- */
-const char* esp_get_idf_version(void);
-
 typedef enum {
     FLASH_SIZE_4M_MAP_256_256 = 0,  /**<  Flash size : 4Mbits. Map : 256KBytes + 256KBytes */
     FLASH_SIZE_2M,                  /**<  Flash size : 2Mbits. Map : 256KBytes */