boot: bootutil: Fix missing packed attributes, add hash size define

Fixes an issue with missing packed attributes on image.h's
structures which would lead to faults on some devices e.g. ARM
Cortex M0, and adds a define for the SHA256 hash size.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>

boot/bootutil/include/bootutil/image.h

@@ -36,6 +36,10 @@
 extern "C" {
 #endif
 
+#ifndef __packed
+#define __packed __attribute__((__packed__))
+#endif
+
 struct flash_area;
 
 #define IMAGE_MAGIC                 0x96f3b83d
@@ -45,6 +49,7 @@ struct flash_area;
 #define IMAGE_TLV_PROT_INFO_MAGIC   0x6908
 
 #define IMAGE_HEADER_SIZE           32
+#define IMAGE_HASH_LEN              32 /* Size of SHA256 TLV hash */
 
 /*
  * Image header flags.
@@ -113,7 +118,7 @@ struct image_version {
     uint8_t iv_minor;
     uint16_t iv_revision;
     uint32_t iv_build_num;
-};
+} __packed;
 
 struct image_dependency {
     uint8_t image_id;                       /* Image index (from 0) */
@@ -135,19 +140,19 @@ struct image_header {
     uint32_t ih_flags;              /* IMAGE_F_[...]. */
     struct image_version ih_ver;
     uint32_t _pad1;
-};
+} __packed;
 
 /** Image TLV header.  All fields in little endian. */
 struct image_tlv_info {
     uint16_t it_magic;
     uint16_t it_tlv_tot;  /* size of TLV area (including tlv_info header) */
-};
+} __packed;
 
 /** Image trailer TLV format. All fields in little endian. */
 struct image_tlv {
     uint16_t it_type;   /* IMAGE_TLV_[...]. */
     uint16_t it_len;    /* Data length (not including TLV header). */
-};
+} __packed;
 
 #define ENCRYPTIONFLAGS (IMAGE_F_ENCRYPTED_AES128 | IMAGE_F_ENCRYPTED_AES256)
 #define IS_ENCRYPTED(hdr) (((hdr)->ih_flags & IMAGE_F_ENCRYPTED_AES128) \