DFU: add init function with area id as parameter

This allows flash_img.c to be used outside of mcuboot scope.
Add new call to not break existing code.

Signed-off-by: Håkon Øye Amundsen <haakon.amundsen@nordicsemi.no>

include/dfu/flash_img.h

@@ -24,6 +24,16 @@ struct flash_img_context {
 #endif
 };
 
+/**
+ * @brief Initialize context needed for writing the image to the flash.
+ *
+ * @param ctx     context to be initialized
+ * @param area_id flash area id of partition where the image should be written
+ *
+ * @return  0 on success, negative errno code on fail
+ */
+int flash_img_init_id(struct flash_img_context *ctx, u8_t area_id);
+
 /**
  * @brief Initialize context needed for writing the image to the flash.
  *