bootutil: add flash device ID accessor

Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>
diff --git a/boot/bootutil/src/bootutil_priv.h b/boot/bootutil/src/bootutil_priv.h
index 52abb76..7676693 100644
--- a/boot/bootutil/src/bootutil_priv.h
+++ b/boot/bootutil/src/bootutil_priv.h
@@ -137,6 +137,18 @@
     return &state->imgs[slot].hdr;
 }
 
+static inline uint8_t
+boot_img_fa_device_id(struct boot_loader_state *state, size_t slot)
+{
+    return state->imgs[slot].sectors[0].fa_device_id;
+}
+
+static inline uint8_t
+boot_scratch_fa_device_id(struct boot_loader_state *state)
+{
+    return state->scratch_sector.fa_device_id;
+}
+
 #ifdef __cplusplus
 }
 #endif