zephyr: Change DT_FLASH_AREA_ macros to FLASH_AREA for multimage

Use the new FLASH_AREA_ macros.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
diff --git a/boot/zephyr/include/target.h b/boot/zephyr/include/target.h
index 3cff9f3..2e0f5fd 100644
--- a/boot/zephyr/include/target.h
+++ b/boot/zephyr/include/target.h
@@ -42,10 +42,8 @@
 #error "Target support is incomplete; cannot build mcuboot."
 #endif
 
-#if ((MCUBOOT_IMAGE_NUMBER == 2) && (!defined(FLASH_AREA_IMAGE_2_OFFSET) || \
-                                     !defined(FLASH_AREA_IMAGE_2_SIZE)   || \
-                                     !defined(FLASH_AREA_IMAGE_3_OFFSET) || \
-                                     !defined(FLASH_AREA_IMAGE_3_SIZE)))
+#if (MCUBOOT_IMAGE_NUMBER == 2) && (!(FLASH_AREA_LABEL_EXISTS(image_2)) || \
+                                     !(FLASH_AREA_LABEL_EXISTS(image_3)))
 #error "Target support is incomplete; cannot build mcuboot."
 #endif