cbfstool: provide buffer_offset()

Instead of people open coding the offset field access within a
struct buffer provide buffer_offset() so that the implementation
can change if needed without high touch in the code base.

Change-Id: I751c7145687a8529ab549d87e412b7f2d1fb90ed
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/13468
Tested-by: build bot (Jenkins)
Reviewed-by: Werner Zeh <werner.zeh@siemens.com>

util/cbfstool/common.h

@@ -62,6 +62,11 @@ static inline size_t buffer_size(const struct buffer *b)
 	return b->size;
 }
 
+static inline size_t buffer_offset(const struct buffer *b)
+{
+	return b->offset;
+}
+
 /*
  * Shrink a buffer toward the beginning of its previous space.
  * Afterward, buffer_delete() remains the means of cleaning it up. */
@@ -125,7 +130,7 @@ static inline void *buffer_get_original_backing(const struct buffer *b)
 {
 	if (!b)
 		return NULL;
-	return b->data - b->offset;
+	return buffer_get(b) - buffer_offset(b);
 }
 
 /* Creates an empty memory buffer with given size.