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_idf_version.h

@@ -0,0 +1,58 @@
+// Copyright 2019 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.
+
+#pragma once
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/** Major version number (X.x.x) */
+#define ESP_IDF_VERSION_MAJOR   3
+/** Minor version number (x.X.x) */
+#define ESP_IDF_VERSION_MINOR   3
+/** Patch version number (x.x.X) */
+#define ESP_IDF_VERSION_PATCH   0
+
+/**
+ * Macro to convert IDF version number into an integer
+ *
+ * To be used in comparisons, such as ESP_IDF_VERSION >= ESP_IDF_VERSION_VAL(4, 0, 0)
+ */
+#define ESP_IDF_VERSION_VAL(major, minor, patch) ((major << 16) | (minor << 8) | (patch))
+
+/**
+ * Current IDF version, as an integer
+ *
+ * To be used in comparisons, such as ESP_IDF_VERSION >= ESP_IDF_VERSION_VAL(4, 0, 0)
+ */
+#define ESP_IDF_VERSION  ESP_IDF_VERSION_VAL(ESP_IDF_VERSION_MAJOR, \
+                                             ESP_IDF_VERSION_MINOR, \
+                                             ESP_IDF_VERSION_PATCH)
+
+/**
+ * Return full IDF version string, same as 'git describe' output.
+ *
+ * @note If you are printing the ESP-IDF version in a log file or other information,
+ * this function provides more information than using the numerical version macros.
+ * For example, numerical version macros don't differentiate between development,
+ * pre-release and release versions, but the output of this function does.
+ *
+ * @return constant string from IDF_VER
+ */
+const char* esp_get_idf_version(void);
+
+#ifdef __cplusplus
+}
+#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 */

components/esp8266/include/internal/esp_system_internal.h

@@ -52,6 +52,16 @@ struct _rtc_sys_info {
                                              if your bootloader is older than v3.2, please don't use this */
 };
 
+/**
+ * @brief System information
+ */
+typedef struct esp_sys_info {
+    uint32_t  version;                                  //!< system version
+    uint32_t  reserved[3];                              //!< reserved data
+} esp_sys_info_t;
+
+_Static_assert(sizeof(esp_sys_info_t) == 16, "esp_sys_info_t should be 16 bytes");
+
 extern struct _rtc_sys_info rtc_sys_info;
 
 /**