Convert CONFIG_VIDEO_LOGO_MAX_SIZE to Kconfig

This converts the following to Kconfig:
   CONFIG_VIDEO_LOGO_MAX_SIZE

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/nitrogen6s1g_defconfig b/configs/nitrogen6s1g_defconfig
index aa28bee..6522a10 100644
--- a/configs/nitrogen6s1g_defconfig
+++ b/configs/nitrogen6s1g_defconfig
@@ -97,5 +97,6 @@
 CONFIG_SPLASH_SCREEN=y
 CONFIG_SPLASH_SCREEN_ALIGN=y
 CONFIG_VIDEO_BMP_GZIP=y
+CONFIG_VIDEO_LOGO_MAX_SIZE=0x600000
 CONFIG_VIDEO_BMP_RLE8=y
 CONFIG_BMP_16BPP=y