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/sandbox_spl_defconfig b/configs/sandbox_spl_defconfig
index 5ce74ed..6d8e827 100644
--- a/configs/sandbox_spl_defconfig
+++ b/configs/sandbox_spl_defconfig
@@ -210,6 +210,7 @@
 CONFIG_VIDEO_SANDBOX_SDL=y
 CONFIG_OSD=y
 CONFIG_SANDBOX_OSD=y
+CONFIG_SPLASH_SCREEN_ALIGN=y
 CONFIG_FS_CBFS=y
 CONFIG_FS_CRAMFS=y
 # CONFIG_SPL_USE_TINY_PRINTF is not set