lenovo: add config ONBOARD_VGA_IS_PRIMARY

Fix for the T4xx and T5xx series.
It does not apply to X2xx/X6x series as those have only
one GPU, which is always connected to the display.

The T6x series needs special care not handled with this patch.

Without ONBOARD_VGA_IS_PRIMARY the onboard GPU would be
deactivated in case a dedicated GPU is found and active,
leaving the system without a working display.

Change-Id: I94d1700e9afb75de83a4f2ed1ff53ba3b0559ae1
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/14031
Tested-by: build bot (Jenkins)
Reviewed-by: Alexander Couzens <lynxis@fe80.eu>
Reviewed-by: Nicolas Reinecke <nr@das-labor.org>
diff --git a/src/mainboard/lenovo/t420s/Kconfig b/src/mainboard/lenovo/t420s/Kconfig
index dc279027..fbfbef5 100644
--- a/src/mainboard/lenovo/t420s/Kconfig
+++ b/src/mainboard/lenovo/t420s/Kconfig
@@ -70,4 +70,8 @@
 	hex
 	default 0x21d2
 
+config ONBOARD_VGA_IS_PRIMARY
+	bool
+	default y
+
 endif # BOARD_LENOVO_T420S