google/oak: define flash size

We never defined the flash size for this board, so the (too small)
default was used. Instead, adopt the size given in depthcharge's fmap
description.

Change-Id: I63782922ee05a9595d6c0de56750460ebb67aec6
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: https://review.coreboot.org/12674
Tested-by: build bot (Jenkins)
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/mainboard/google/oak/Kconfig

@@ -23,6 +23,7 @@ config BOARD_SPECIFIC_OPTIONS
 	def_bool y
 	select SOC_MEDIATEK_MT8173
 	select BOARD_ID_AUTO
+	select BOARD_ROMSIZE_KB_4096
 	select CHROMEOS_VBNV_EC
 	select EC_GOOGLE_CHROMEEC
 	select EC_GOOGLE_CHROMEEC_SPI