mainboard/google/kahlee: Enable EC firmware update screen

Grunt takes a few seconds to update the EC, so display a notification
screen while that's happening.

BUG=b:113286040
TEST=Boot Grunt with old EC firmware, see update screen

Change-Id: I95fc4d3430bac66c09f57a4d34abde08752e5f0e
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/28374
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/mainboard/google/kahlee/Kconfig

@@ -98,6 +98,7 @@ config ONBOARD_VGA_IS_PRIMARY
 config VBOOT
 	select EC_GOOGLE_CHROMEEC_SWITCHES
 	select VBOOT_LID_SWITCH
+	select VBOOT_EC_SLOW_UPDATE if VBOOT_EC_SOFTWARE_SYNC
 
 config VBOOT_VBNV_OFFSET
 	hex