colibri-imx6ull: fix splash screen logo drawing

Configure white on black for video console.

Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>
diff --git a/configs/colibri-imx6ull_defconfig b/configs/colibri-imx6ull_defconfig
index ce145cd..3292fcc 100644
--- a/configs/colibri-imx6ull_defconfig
+++ b/configs/colibri-imx6ull_defconfig
@@ -91,3 +91,4 @@
 CONFIG_DM_VIDEO=y
 CONFIG_OF_LIBFDT_OVERLAY=y
 CONFIG_FDT_FIXUP_PARTITIONS=y
+CONFIG_SYS_WHITE_ON_BLACK=y