Call device binding for DT_FLASH_DEV_NAME only if it is defined

Signed-off-by: Rajavardhan Gundi <rajavardhan.gundi@intel.com>

boot/zephyr/main.c

@@ -108,11 +108,13 @@ void main(void)
 
     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