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>
diff --git a/src/lib/coreboot_table.c b/src/lib/coreboot_table.c
index 091ceaf..6967217 100644
--- a/src/lib/coreboot_table.c
+++ b/src/lib/coreboot_table.c
@@ -445,6 +445,19 @@
 	return (unsigned long)rec + rec->size;
 }
 
+size_t write_coreboot_forwarding_table(uintptr_t entry, uintptr_t target)
+{
+	struct lb_header *head;
+
+	printk(BIOS_DEBUG, "Writing table forward entry at 0x%p\n",
+		(void *)entry);
+
+	head = lb_table_init(entry);
+	lb_forward(head, (struct lb_header*)target);
+
+	return (uintptr_t)lb_table_fini(head) - entry;
+}
+
 unsigned long write_coreboot_table(
 	unsigned long low_table_start, unsigned long low_table_end,
 	unsigned long rom_table_start __unused, unsigned long rom_table_end)