write_tables: return a pointer to the table

The write_tables function was void. It is a bit more
useful for loading payloads from the romstage
if it returns a pointer to the table it creates.

Change-Id: I6eeaf3e16bcbaf1e7ec3eada8026c466d2fb6f5a
Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-on: https://review.coreboot.org/27537
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/include/boot/tables.h

@@ -6,8 +6,9 @@
 /*
  * Write architecture specific tables as well as the common
  * coreboot table.
+ * Returns a pointer to the table or NULL on error.
  */
-void write_tables(void);
+void *write_tables(void);
 
 /*
  * Allow per-architecture table writes called from write_tables(). The