Convert CONFIG_SPLASH_SCREEN et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SPLASH_SCREEN
   CONFIG_SPLASH_SCREEN_ALIGN
   CONFIG_SPLASHIMAGE_GUARD
   CONFIG_SPLASH_SOURCE

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/nitrogen6q_defconfig b/configs/nitrogen6q_defconfig
index 0846b40..637b2bb 100644
--- a/configs/nitrogen6q_defconfig
+++ b/configs/nitrogen6q_defconfig
@@ -82,3 +82,5 @@
 CONFIG_DM_VIDEO=y
 CONFIG_SYS_WHITE_ON_BLACK=y
 CONFIG_VIDEO_IPUV3=y
+CONFIG_SPLASH_SCREEN=y
+CONFIG_SPLASH_SCREEN_ALIGN=y