google/gru: Shrink RW_ELOG region to 4KB

Since there's currently a limitation in coreboot's code that prevents
more than 4KB to be used by the eventlog anyway, this patch shrinks the
available RW_ELOG area in the FMAP for Gru down to 4KB. This may prove
prudent later if we ever resolve that limitation, so that tools can rely
on the area in the FMAP being the same as the area actually used by the
read-only firmware code on these boards.

BRANCH=gru
BUG=chrome-os-partner:55593
TEST=Booted Kevin, confirmed that eventlog got written normally. Ran a
reboot loop to exhaust eventlog space, confirmed that the shrink code
kicks in as expected before reaching 4KB.

Change-Id: I3c55d836c72486665a19783fe98ce9e0df174b6d
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: 05efb82ca00703fd92d925ebf717738e37295c18
Original-Change-Id: Ia2617681f9394e953f5beb4abf419fe8d97e6d3e
Original-Signed-off-by: Julius Werner <jwerner@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/384585
Original-Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Original-Reviewed-by: Simon Glass <sjg@google.com>
Reviewed-on: https://review.coreboot.org/16715
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>

src/mainboard/google/gru/chromeos.fmd

@@ -20,7 +20,8 @@ FLASH@0x0 0x800000 {
 		FW_MAIN_B(CBFS)@0x2000 0xe5f00
 		RW_FWID_B@0xe7f00 0x100
 	}
-	RW_ELOG@0x5d8000 0x8000
+	RW_ELOG@0x5d8000 0x1000
+	RW_UNUSED@0x5d9000 0x7000
 	RW_SHARED@0x5e0000 0x10000 {
 		SHARED_DATA@0x0 0x10000
 	}