bootutil: Small cleanup in image.h

#define STRUCT_PACKED struct __attribute__((__packed__))

#endif

struct flash_area;

#define IMAGE_MAGIC                 0x96f3b83d

#define IMAGE_MAGIC_V1              0x96f3b83c

#define IMAGE_MAGIC_NONE            0xffffffff

_Static_assert(sizeof(struct image_header) == IMAGE_HEADER_SIZE,

               "struct image_header not required size");

struct enc_key_data;

struct boot_loader_state;

struct flash_area;

fih_ret bootutil_img_validate(struct boot_loader_state *state,

                              struct image_header *hdr,

                              const struct flash_area *fap,