Add macro to get TLVs offset from header

TLV offset was determined by manually adding the header and image
size in many places; this makes the addition into a macro receiving
a image_header to ease future changes.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/bootutil/src/image_validate.c b/boot/bootutil/src/image_validate.c
index 01efbcb..a496bc4 100644
--- a/boot/bootutil/src/image_validate.c
+++ b/boot/bootutil/src/image_validate.c
@@ -69,6 +69,7 @@
 #if (BOOT_IMAGE_NUMBER == 1) || !defined(MCUBOOT_ENC_IMAGES)
     (void)enc_state;
     (void)image_index;
+    (void)hdr_size;
 #endif
 
     bootutil_sha256_init(&sha256_ctx);
@@ -89,7 +90,7 @@
 
     /* Hash is computed over image header and image itself. */
     hdr_size = hdr->ih_hdr_size;
-    size = hdr->ih_img_size + hdr_size;
+    size = BOOT_TLV_OFF(hdr);
 
 #if (MCUBOOT_IMAGE_NUMBER > 1)
     /* If dependency TLVs are present then the TLV info header and the
@@ -231,7 +232,7 @@
     }
 
     /* The TLVs come after the image. */
-    off = hdr->ih_img_size + hdr->ih_hdr_size;
+    off = BOOT_TLV_OFF(hdr);
 
     rc = flash_area_read(fap, off, &info, sizeof(info));
     if (rc) {