imagetool: make the image_save_datafile() available to all image types

Move the image_save_datafile() function from an U-Multi specific file
(default_image.c) to a file common to all image types (image.c). And rename it
to genimg_save_datafile(), to make clear it is useful for any image type.

Signed-off-by: Guilherme Maciel Ferreira <guilherme.maciel.ferreira@gmail.com>
diff --git a/common/image.c b/common/image.c
index ad7a46d..a911aa9 100644
--- a/common/image.c
+++ b/common/image.c
@@ -756,7 +756,7 @@
  * genimg_get_image - get image from special storage (if necessary)
  * @img_addr: image start address
  *
- * genimg_get_image() checks if provided image start adddress is located
+ * genimg_get_image() checks if provided image start address is located
  * in a dataflash storage. If so, image is moved to a system RAM memory.
  *
  * returns: