x86: Move coreboot-table detection into common code

To support detecting booting from coreboot, move the code which locates
the coreboot tables into a common place. Adjust the algorithm slightly to
use a word comparison instead of string, since it is faster.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: correct the comments to 960KB]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>

arch/x86/include/asm/coreboot_tables.h

@@ -343,4 +343,11 @@ void *high_table_malloc(size_t bytes);
  */
 void write_coreboot_table(u32 addr, struct memory_area *cfg_tables);
 
+/**
+ * locate_coreboot_table() - Try to find coreboot tables at standard locations
+ *
+ * @return address of table that was found, or -ve error number
+ */
+long locate_coreboot_table(void);
+
 #endif