elog: Eliminate CONFIG_ELOG_FULL_THRESHOLD and CONFIG_ELOG_SHRINK_SIZE

These Kconfig options provided a level of configurability that is
almost never necessary, so they are being moved into ordinary
preprocessor defines in elog_internal.h. The new threshold to
trigger shrinking is relative to the number of additional
(maximum-size) events that can fit, and the new target
post-shrink size is a percentage of the total ELOG area size.

BUG=chromium:467820
TEST=Add loop at the end of elog_init() that fills the ELOG area
to just below full_threshold with dummy events. Observe
successful shrinkage when the next event is logged.
BRANCH=None

Change-Id: I414c4955a2d819d112ae4f0c7d3571576f732336
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: ce439361e3954a2bf5186292f96936329171cf56
Original-Change-Id: I926097f86262888dcdd47d73fba474bb2e19856a
Original-Signed-off-by: Sol Boucher <solb@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/260501
Original-Reviewed-by: Duncan Laurie <dlaurie@chromium.org>
Original-Reviewed-by: Stefan Reinauer <reinauer@chromium.org>
Reviewed-on: http://review.coreboot.org/9869
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/drivers/elog/elog_internal.h

@@ -31,6 +31,8 @@ struct elog_header {
 /* ELOG related constants */
 #define ELOG_SIGNATURE			0x474f4c45  /* 'ELOG' */
 #define ELOG_VERSION			1
+#define ELOG_MIN_AVAILABLE_ENTRIES	2  /* Shrink when this many can't fit */
+#define ELOG_SHRINK_PERCENTAGE		25 /* Percent of total area to remove */
 
 /* SMBIOS event log header */
 struct event_header {