阅读ram.bin 机制，添加一些注释

inc/utils/iot_img_hdr.h

@@ -258,7 +258,7 @@ typedef struct
     uint8_t     layoutIdx;
     /* reserved */
     uint8_t     resv[3];
-    /* reserved */
+    /* 这个可能会用来存放压缩前img的crc */
     uint32_t    fwCRC;
     /* sha256 for payload */
     uint8_t     sha256[SHA256_SIZE];
@@ -287,12 +287,12 @@ typedef struct
     /* Psram/DDR size, reference for sizeType. */
     uint8_t     psramSize;
     /* Header version. reference for hdrVersion. */
-    uint8_t     hdrVer;
+    uint8_t     hdrVer; // imgheader版本,V1和V0位置相同
     uint32_t    imgVer;
     /* CRC32 for 'payload + pad for aligning' */
     uint32_t    imgCRC;
     /* fixed, 0xa4e49a17 */
-    uint32_t    guard;
+    uint32_t    guard; // 通过这个值来确定是不是imgheader,V1和V0位置相同
     /* flash layout index */
     uint8_t     layoutIdx;
     /* reserved */