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/s5p4418_nanopi2_defconfig b/configs/s5p4418_nanopi2_defconfig
index 96a5c66..3b21691 100644
--- a/configs/s5p4418_nanopi2_defconfig
+++ b/configs/s5p4418_nanopi2_defconfig
@@ -52,4 +52,7 @@
 CONFIG_VIDEO_NX_RGB=y
 CONFIG_VIDEO_NX_LVDS=y
 CONFIG_VIDEO_NX_HDMI=y
+CONFIG_SPLASH_SCREEN=y
+CONFIG_SPLASH_SCREEN_ALIGN=y
+CONFIG_SPLASH_SOURCE=y
 CONFIG_ERRNO_STR=y