bootutil: add accessor for write_sz

This finishes making the struct contents opaque.

Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>
diff --git a/boot/bootutil/src/bootutil_priv.h b/boot/bootutil/src/bootutil_priv.h
index 456a6a2..7603bd0 100644
--- a/boot/bootutil/src/bootutil_priv.h
+++ b/boot/bootutil/src/bootutil_priv.h
@@ -139,6 +139,7 @@
 /* These are macros so they can be used as lvalues. */
 #define BOOT_IMG_AREA(state, slot) ((state)->imgs[(slot)].area)
 #define BOOT_SCRATCH_AREA(state) ((state)->scratch_area)
+#define BOOT_WRITE_SZ(state) ((state)->write_sz)
 
 static inline struct image_header*
 boot_img_hdr(struct boot_loader_state *state, size_t slot)