oak: Select VBOOT_EC_SLOW_UPDATE Kconfig option

All current Oak boards have PD chips with update speeds that range from
slow (Oak) to "OMG it's so awfully slow I could make a cup of coffee and
it would still not be done" (Elm). Set the flag that enables the "Your
system is applying a critical update. Please don't turn it off." message
on EC software sync so that our users don't accidentally carry it back
to the store and demand a refund while it's still not done booting.

BRANCH=None
BUG=chrome-os-partner:51145
TEST=Booted Oak in normal mode with a new EC-RW image. Confirmed that I
saw the magic screen.

Change-Id: I000eab36d26b61b25d1f0da505f02ced15457255
Signed-off-by: Martin Roth <martinroth@chromium.org>
Original-Commit-Id: 274644b
Original-Change-Id: I64ba698985d5fbcf2b94115df72b70a5319106ac
Original-Signed-off-by: Julius Werner <jwerner@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/348787
Original-Reviewed-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-on: https://review.coreboot.org/15114
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/mainboard/google/oak/Kconfig

@@ -36,6 +36,7 @@ config BOARD_SPECIFIC_OPTIONS
 config CHROMEOS
 	select CHROMEOS_VBNV_FLASH
 	select EC_SOFTWARE_SYNC
+	select VBOOT_EC_SLOW_UPDATE
 	select VBOOT_OPROM_MATTERS
 	select VIRTUAL_DEV_SWITCH