elog: Get rid of the staging_header variable.

The header is at the start of the log. There's no reason to either keep a
seperate pointer to it, or to keep a copy of it in some other bit of memory.

Built and booted on Link and used 'mosys eventlog list' to list the
contents of the log. Ran

for x in $(seq 1 2000); do
  cat elog.event.kernel_clean > /sys/firmware/gsmi/append_to_eventlog;
done

And ran mosys eventlog list again to verify that the log had been shrunk
correctly.

Change-Id: I2afcd52c0ce5bbb662ac56f2895cdbea28d5c2ce
Signed-off-by: Gabe Black <gabeblack@google.com>
Reviewed-on: https://gerrit.chromium.org/gerrit/49304
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>
Commit-Queue: Gabe Black <gabeblack@chromium.org>
Tested-by: Gabe Black <gabeblack@chromium.org>
Reviewed-on: http://review.coreboot.org/4239
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/drivers/elog/elog_internal.h

@@ -77,7 +77,6 @@ struct elog_descriptor {
 	elog_area_state		area_state;
 	elog_header_state	header_state;
 	elog_event_buffer_state	event_buffer_state;
-	struct elog_header	*staging_header;
 	void			*backing_store;
 	u8			*data;
 	u32			flash_base;