zephyr: Replace DT_FLASH_DEV_NAME with DT_CHOSEN_ZEPHYR_FLASH_CONTROLLER_LABEL

As upstream zephyr is phasing out DT_FLASH defines for non-partition
usage replace DT_FLASH_DEV_NAME with
DT_CHOSEN_ZEPHYR_FLASH_CONTROLLER_LABEL.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>
diff --git a/boot/zephyr/include/target.h b/boot/zephyr/include/target.h
index 57a412b..794f40f 100644
--- a/boot/zephyr/include/target.h
+++ b/boot/zephyr/include/target.h
@@ -28,7 +28,7 @@
 /*
  * Sanity check the target support.
  */
-#if (!defined(CONFIG_XTENSA) && !defined(DT_FLASH_DEV_NAME)) || \
+#if (!defined(CONFIG_XTENSA) && !defined(DT_CHOSEN_ZEPHYR_FLASH_CONTROLLER_LABEL)) || \
     (defined(CONFIG_XTENSA) && !defined(DT_JEDEC_SPI_NOR_0_LABEL)) || \
     !defined(FLASH_ALIGN) ||                  \
     !defined(DT_FLASH_AREA_IMAGE_0_OFFSET) || \