mb/google/slippy: Add a VBT for all variants

The vbt was extracted from the option rom found on stock images.
The vbt.bin is the same across all variants.

The VBT has a modified BDB block 43, the 'Backlight info block' such
that the inverter type for the panel in use is set to
2 (BDB_BACKLIGHT_TYPE_PWM) instead of 0 (BDB_BACKLIGHT_TYPE_NONE).
This only seems to matter on Windows, as without it changing the
backlight duty cycle does not work.

Change-Id: I82c72c561e1058e0b77d80baf330b64f7c6b08e3
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/30487
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-by: Tristan Corrick <tristan@corrick.kiwi>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/google/slippy/Kconfig

@@ -15,6 +15,7 @@ config BOARD_GOOGLE_BASEBOARD_SLIPPY
 	select MAINBOARD_HAS_TPM1
 	select INTEL_INT15
 	select MAINBOARD_HAS_LIBGFXINIT
+	select INTEL_GMA_HAVE_VBT
 
 if BOARD_GOOGLE_BASEBOARD_SLIPPY
 
@@ -65,4 +66,10 @@ config VGA_BIOS_FILE
 	string
 	default "pci8086,0406.rom"
 
+# Override the default variant behavior, since the data.vbt is the same
+# for all variants.
+config INTEL_GMA_VBT_FILE
+	default "src/mainboard/$(MAINBOARDDIR)/data.vbt"
+
+
 endif # BOARD_GOOGLE_BASEBOARD_SLIPPY