fix zephyr target: corrupted build after zephyr PR #11180

zephyrproject-rtos/zephyr#11180: Zephyr target was corrupted as
recently zephyr's device tree started adding DT_ prefix in
generated labels.

This path aligns flash name macro used.


Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
diff --git a/boot/zephyr/main.c b/boot/zephyr/main.c
index 1f097a6..d773dde 100644
--- a/boot/zephyr/main.c
+++ b/boot/zephyr/main.c
@@ -108,8 +108,8 @@
 
     os_heap_init();
 
-    if (!flash_device_get_binding(FLASH_DEV_NAME)) {
-        BOOT_LOG_ERR("Flash device %s not found", 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)
             ;
     }