zephyr: allow dynamic numeration of flash_areas

Zephyr flash_map reworks caused that areas id exact number are
assigned dynamically.

This patch i counterpart to
https://github.com/zephyrproject-rtos/zephyr/pull/8837

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
diff --git a/boot/zephyr/include/sysflash/sysflash.h b/boot/zephyr/include/sysflash/sysflash.h
index e08f9eb..0fef026 100644
--- a/boot/zephyr/include/sysflash/sysflash.h
+++ b/boot/zephyr/include/sysflash/sysflash.h
@@ -3,8 +3,10 @@
 #ifndef __SYSFLASH_H__
 #define __SYSFLASH_H__
 
-#define FLASH_AREA_IMAGE_0 1
-#define FLASH_AREA_IMAGE_1 2
-#define FLASH_AREA_IMAGE_SCRATCH 3
+#include <generated_dts_board.h>
+
+#define FLASH_AREA_IMAGE_0 DT_FLASH_AREA_IMAGE_0_ID
+#define FLASH_AREA_IMAGE_1 DT_FLASH_AREA_IMAGE_1_ID
+#define FLASH_AREA_IMAGE_SCRATCH DT_FLASH_AREA_IMAGE_SCRATCH_ID
 
 #endif /* __SYSFLASH_H__ */