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/mx23evk_defconfig b/configs/mx23evk_defconfig
index 227c627..51f4a0f 100644
--- a/configs/mx23evk_defconfig
+++ b/configs/mx23evk_defconfig
@@ -38,4 +38,5 @@
 CONFIG_USB_EHCI_HCD=y
 CONFIG_USB_STORAGE=y
 CONFIG_VIDEO=y
+CONFIG_SPLASH_SCREEN=y
 CONFIG_OF_LIBFDT=y