mb/lenovo/x1_carbon_gen1: add vbt

Extracted from live running Thinkpad x1 carbon gen1 with vendor firmware.
Thanks to Igor Lee.

Change-Id: Id59517d9040c98e67a42fbce537f42f6b0c6db2d
Signed-off-by: Alexander Couzens <lynxis@fe80.eu>
Tested-by: Igor Lee <getrun@gmail.com>
Reviewed-on: https://review.coreboot.org/27782
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/mainboard/lenovo/x1_carbon_gen1/Kconfig

@@ -22,6 +22,7 @@ config BOARD_SPECIFIC_OPTIONS # dummy
 	select MAINBOARD_HAS_TPM1
 	select MAINBOARD_HAS_LIBGFXINIT
 	select GFX_GMA_INTERNAL_IS_LVDS
+	select INTEL_GMA_HAVE_VBT
 
 	# Workaround for EC/KBC IRQ1.
 	select SERIRQ_CONTINUOUS_MODE