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/sei610_defconfig b/configs/sei610_defconfig
index 39d36b1..43932b4 100644
--- a/configs/sei610_defconfig
+++ b/configs/sei610_defconfig
@@ -78,4 +78,6 @@
 CONFIG_SYS_WHITE_ON_BLACK=y
 CONFIG_VIDEO_MESON=y
 CONFIG_VIDEO_DT_SIMPLEFB=y
+CONFIG_SPLASH_SCREEN=y
+CONFIG_SPLASH_SCREEN_ALIGN=y
 CONFIG_OF_LIBFDT_OVERLAY=y