lib: add helper for constructing coreboot forwarding table

The x86 architecture needs to add a forwarding table to
the real coreboot table. Provide a helper function to do
this for aligning the architectures on a common
write_tables() implementation.

Change-Id: I9a2875507e6260679874a654ddf97b879222d44e
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/14433
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/include/boot/coreboot_tables.h

@@ -9,6 +9,12 @@ unsigned long write_coreboot_table(
 	unsigned long low_table_start, unsigned long low_table_end,
 	unsigned long rom_table_start, unsigned long rom_table_end);
 
+/*
+ * Write forwarding table of target address at entry address returning size
+ * of table written.
+ */
+size_t write_coreboot_forwarding_table(uintptr_t entry, uintptr_t target);
+
 void fill_lb_gpios(struct lb_gpios *gpios);
 void lb_add_gpios(struct lb_gpios *gpios, const struct lb_gpio *gpio_table,
 		  size_t count);