Call device binding for DT_FLASH_DEV_NAME only if it is defined

Signed-off-by: Rajavardhan Gundi <rajavardhan.gundi@intel.com>
diff --git a/boot/zephyr/main.c b/boot/zephyr/main.c
index d773dde..09286b6 100644
--- a/boot/zephyr/main.c
+++ b/boot/zephyr/main.c
@@ -108,11 +108,13 @@
 
     os_heap_init();
 
+#ifdef 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);
         while (1)
             ;
     }
+#endif
 
 #ifdef CONFIG_MCUBOOT_SERIAL