bootutil_public: Make  boot_read_swap_state declaration public

The function itself is not static within bootutil and may be used
to operate on flash areas, for which the flash_area object has
already been obtained.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
diff --git a/boot/bootutil/include/bootutil/bootutil_public.h b/boot/bootutil/include/bootutil/bootutil_public.h
index 3ff7946..8843082 100644
--- a/boot/bootutil/include/bootutil/bootutil_public.h
+++ b/boot/bootutil/include/bootutil/bootutil_public.h
@@ -4,7 +4,7 @@
  * Copyright (c) 2017-2019 Linaro LTD
  * Copyright (c) 2016-2019 JUUL Labs
  * Copyright (c) 2019-2021 Arm Limited
- * Copyright (c) 2020 Nordic Semiconductor ASA
+ * Copyright (c) 2020-2021 Nordic Semiconductor ASA
  *
  * Original license:
  *
@@ -223,14 +223,26 @@
 /**
  * @brief Read the image swap state
  *
- * @param flash_area_id Id of flash parttition from which trailer will be read.
- * @param state Struture for holding swap state.
+ * @param flash_area_id id of flash partition from which state will be read;
+ * @param state pointer to structure for storing swap state.
  *
- * @return 0 on success, nonzero errno code on fail.
+ * @return 0 on success; non-zero error code on failure;
  */
 int
 boot_read_swap_state_by_id(int flash_area_id, struct boot_swap_state *state);
 
+/**
+ * @brief Read the image swap state
+ *
+ * @param fa pointer to flash_area object;
+ * @param state pointer to structure for storing swap state.
+ *
+ * @return 0 on success; non-zero error code on failure.
+ */
+int
+boot_read_swap_state(const struct flash_area *fa,
+                     struct boot_swap_state *state);
+
 #define BOOT_MAGIC_ARR_SZ \
     (sizeof boot_img_magic / sizeof boot_img_magic[0])