EC: Prepare to read and log last post code from previous boot

(elog portion, support in EC code pending)

- Use a new EC command to read the last post code
from the previous boot
- If the post code is not well-known final boot
or resume code then log it

Change-Id: Id6249e9a182243eb87c777edd56f48de72125e77
Signed-off-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-on: http://review.coreboot.org/1703
Reviewed-by: Marc Jones <marcj303@gmail.com>
Tested-by: build bot (Jenkins)

src/include/elog.h

@@ -109,6 +109,9 @@ struct elog_event_data_wake {
 /* Management Engine Events */
 #define ELOG_TYPE_MANAGEMENT_ENGINE       0xa2
 
+/* Last post code from previous boot */
+#define ELOG_TYPE_LAST_POST_CODE          0xa3
+
 extern int elog_init(void);
 extern int elog_clear(void);
 extern void elog_add_event_raw(u8 event_type, void *data, u8 data_size);