Fix verification with sparse layouts

The full verification step was not accounting for sparse layouts.
Instead of the old contents, combine_image_by_layout() implicitly
assumed the new contents for unspecified regions.

Change-Id: I44e0cea621f2a3d4dc70fa7e93c52ed95e54014a
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/30372
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

layout.h

@@ -60,5 +60,6 @@ struct single_layout {
 struct flashrom_layout *get_global_layout(void);
 
 int process_include_args(struct flashrom_layout *);
+const struct romentry *layout_next_included_region(const struct flashrom_layout *, chipoff_t);
 
 #endif				/* !__LAYOUT_H__ */