lenovo/x60: Enable VESA framebuffer mode (native graphics)

At present, no option exists for "Keep VESA framebuffer", which
means that text-mode will be used. Add the appropriate Kconfig
option.

Change-Id: Iaed07eba6d9288c857f7e7a0b0be1107071e49e5
Signed-off-by: Francis Rowe <info@gluglug.org.uk>
Reviewed-on: http://review.coreboot.org/10553
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <gaumless@gmail.com>

src/mainboard/lenovo/x60/Kconfig

@@ -24,6 +24,7 @@ config BOARD_SPECIFIC_OPTIONS # dummy
 	select HAVE_ACPI_RESUME
 	select USE_OPTION_TABLE
 	select MAINBOARD_HAS_NATIVE_VGA_INIT
+	select MAINBOARD_HAS_NATIVE_VGA_INIT_TEXTMODECFG
 	select H8_DOCK_EARLY_INIT
 	select DRIVERS_LENOVO_WACOM
 	select INTEL_EDID