mb/lenovo/haswell: Hook up variants Makefile

Change-Id: I36091118d98f71dc4141aca4e45858a22d519a9b
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/66107
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/mainboard/lenovo/haswell/Makefile.inc

@@ -3,3 +3,5 @@ romstage-y += variants/$(VARIANT_DIR)/romstage.c
 
 ramstage-y += mainboard.c
 ramstage-$(CONFIG_MAINBOARD_USE_LIBGFXINIT) += variants/$(VARIANT_DIR)/gma-mainboard.ads
+
+subdirs-y += variants/$(VARIANT_DIR)