zephyr/boot_serial_extension: Switch to using FIXED_PARTITION_

Switching from FLASH_AREA_ to FIXED_PARTITION_ macros.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
diff --git a/boot/zephyr/boot_serial_extensions.c b/boot/zephyr/boot_serial_extensions.c
index 5ebb902..de6e923 100644
--- a/boot/zephyr/boot_serial_extensions.c
+++ b/boot/zephyr/boot_serial_extensions.c
@@ -28,12 +28,12 @@
 
     const struct flash_area *fa;
 
-    rc = flash_area_open(FLASH_AREA_ID(storage), &fa);
+    rc = flash_area_open(FIXED_PARTITION_ID(storage_partition), &fa);
 
     if (rc < 0) {
         BOOT_LOG_ERR("failed to open flash area");
     } else {
-        rc = flash_area_erase(fa, 0, FLASH_AREA_SIZE(storage));
+        rc = flash_area_erase(fa, 0, flash_area_get_size(fa));
         if (rc < 0) {
             BOOT_LOG_ERR("failed to erase flash area");
         }
diff --git a/boot/zephyr/flash_map_extended.c b/boot/zephyr/flash_map_extended.c
index 180909c..39bc652 100644
--- a/boot/zephyr/flash_map_extended.c
+++ b/boot/zephyr/flash_map_extended.c
@@ -94,18 +94,18 @@
     switch (image_id) {
     case 0:
     case 1:
-        return FLASH_AREA_ID(image_0);
-#if DT_HAS_FIXED_PARTITION_LABEL(image_1)
+        return FIXED_PARTITION_ID(slot0_partition);
+#if FIXED_PARTITION_EXISTS(slot1_partition)
     case 2:
-        return FLASH_AREA_ID(image_1);
+        return FIXED_PARTITION_ID(slot1_partition);
 #endif
-#if DT_HAS_FIXED_PARTITION_LABEL(image_2)
+#if FIXED_PARTITION_EXISTS(slot2_partition)
     case 3:
-        return FLASH_AREA_ID(image_2);
+        return FIXED_PARTITION_ID(slot2_partition);
 #endif
-#if DT_HAS_FIXED_PARTITION_LABEL(image_3)
+#if FIXED_PARTITION_EXISTS(slot3_partition)
     case 4:
-        return FLASH_AREA_ID(image_3);
+        return FIXED_PARTITION_ID(slot3_partition);
 #endif
     }
     return -EINVAL;