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/bootutil_priv.h b/boot/bootutil/src/bootutil_priv.h
index 7ae6103..a2e8f75 100644
--- a/boot/bootutil/src/bootutil_priv.h
+++ b/boot/bootutil/src/bootutil_priv.h
@@ -275,6 +275,7 @@
 #define BOOT_SCRATCH_AREA(state) ((state)->scratch.area)
 #define BOOT_WRITE_SZ(state) ((state)->write_sz)
 #define BOOT_SWAP_TYPE(state) ((state)->swap_type[BOOT_CURR_IMG(state)])
+#define BOOT_TLV_OFF(hdr) ((hdr)->ih_hdr_size + (hdr)->ih_img_size)
 
 static inline struct image_header*
 boot_img_hdr(struct boot_loader_state *state, size_t slot)