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/main.c b/boot/zephyr/main.c
index 978d517..ce62851 100644
--- a/boot/zephyr/main.c
+++ b/boot/zephyr/main.c
@@ -277,9 +277,10 @@
 
     ZEPHYR_BOOT_LOG_START();
 
-#if (!defined(CONFIG_XTENSA) && defined(DT_FLASH_DEV_NAME))
-    if (!flash_device_get_binding(DT_FLASH_DEV_NAME)) {
-        BOOT_LOG_ERR("Flash device %s not found", DT_FLASH_DEV_NAME);
+#if (!defined(CONFIG_XTENSA) && defined(DT_CHOSEN_ZEPHYR_FLASH_CONTROLLER_LABEL))
+    if (!flash_device_get_binding(DT_CHOSEN_ZEPHYR_FLASH_CONTROLLER_LABEL)) {
+        BOOT_LOG_ERR("Flash device %s not found",
+		     DT_CHOSEN_ZEPHYR_FLASH_CONTROLLER_LABEL);
         while (1)
             ;
     }