bootutil: Replace BUILD_ASSERT_MSG() with BUILD_ASSERT()

Replay of reverted to early committed patch 6d417c9:

Replace BUILD_ASSERT_MSG() with BUILD_ASSERT()
after merging BUILD_ASSERT() and BUILD_ASSERT_MSG().

Signed-off-by: Oleg Zhurakivskyy <oleg.zhurakivskyy@intel.com>
Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
diff --git a/boot/bootutil/include/bootutil/image.h b/boot/bootutil/include/bootutil/image.h
index 4a9fedc..2eb519f 100644
--- a/boot/bootutil/include/bootutil/image.h
+++ b/boot/bootutil/include/bootutil/image.h
@@ -134,8 +134,8 @@
     ((fap)->fa_id == FLASH_AREA_IMAGE_SECONDARY(idx) && IS_ENCRYPTED(hdr))
 
 #ifdef __ZEPHYR__
-BUILD_ASSERT_MSG(sizeof(struct image_header) == IMAGE_HEADER_SIZE,
-               "struct image_header not required size");
+BUILD_ASSERT(sizeof(struct image_header) == IMAGE_HEADER_SIZE,
+	     "struct image_header not required size");
 #else
 _Static_assert(sizeof(struct image_header) == IMAGE_HEADER_SIZE,
                "struct image_header not required size");