colibri-imx6ull: fix splash screen logo drawing

Configure white on black for video console.
Signed-off-by: NIgor Opaniuk <igor.opaniuk@toradex.com>

configs/colibri-imx6ull_defconfig

@@ -91,3 +91,4 @@ CONFIG_USB_GADGET_DOWNLOAD=y
 CONFIG_DM_VIDEO=y
 CONFIG_OF_LIBFDT_OVERLAY=y
 CONFIG_FDT_FIXUP_PARTITIONS=y
+CONFIG_SYS_WHITE_ON_BLACK=y